(CNN) It began with the arrests of a handful of children in 2011. Since then, it's exploded into the biggest humanitarian crisis since the Second World War.

It all starts with graffiti

Damascus skies erupt with anti-aircraft fire as the US and its allies launch an attack on Syria's capital early on April 14, 2018. US President Donald Trump announced airstrikes in retaliation for Syria's alleged use of chemical weapons. Trump says the strikes are part of a sustained military response, in coordination with France and the United Kingdom.

Damascus skies erupt with anti-aircraft fire as the US and its allies launch an attack on Syria's capital early on April 14, 2018. US President Donald Trump announced airstrikes in retaliation for Syria's alleged use of chemical weapons. Trump says the strikes are part of a sustained military response, in coordination with France and the United Kingdom.

Bodies lie on the ground in the rebel-held city of Douma, Syria, on April 8, 2018. According to activist groups, helicopters dropped barrel bombs filled with toxic gas on Douma, which has been the focus of a renewed government offensive that launched in mid-February. The Syrian government and its key ally, Russia, vehemently denied involvement and accused rebel groups of fabricating the attack to hinder the army's advances and provoke international military intervention.

A child receives medical treatment after a village was attacked in the rebel-held Eastern Ghouta region on February 25, 2018. Several people were treated for exposure to chlorine gas, opposition groups said, as airstrikes and artillery fire from the regime continued. CNN was unable to independently verify claims that chlorine was used as a weapon.

A child receives medical treatment after a village was attacked in the rebel-held Eastern Ghouta region on February 25, 2018. Several people were treated for exposure to chlorine gas, opposition groups said, as airstrikes and artillery fire from the regime continued. CNN was unable to independently verify claims that chlorine was used as a weapon.

A member of the Syrian pro-regime forces fires a machine gun as a comrade holds his feeding ammunition belt on November 11, 2017. It was during an advance toward rebel-held positions west of Aleppo.

A member of the Syrian pro-regime forces fires a machine gun as a comrade holds his feeding ammunition belt on November 11, 2017. It was during an advance toward rebel-held positions west of Aleppo.

Residents of the war-torn city of Douma break their Ramadan fast on June 18, 2017.

Residents of the war-torn city of Douma break their Ramadan fast on June 18, 2017.

Members of the UN Security Council raise their hands on April 12, 2017, as they vote in favor of a draft resolution that condemned the reported use of chemical weapons in Syria.

Members of the UN Security Council raise their hands on April 12, 2017, as they vote in favor of a draft resolution that condemned the reported use of chemical weapons in Syria.

This photo, provided by the activist Idlib Media Center, shows dead children after a suspected chemical attack in the rebel-held city of Khan Sheikhoun on April 4, 2017. Dozens of people were killed, according to multiple activist groups. The United States responded a few days later by launching between 50-60 Tomahawk missiles at a Syrian government airbase. US officials said the base was home to warplanes that carried out the chemical attack. Syria has repeatedly denied it had anything to do with the attack.

This photo, provided by the activist Idlib Media Center, shows dead children after a suspected chemical attack in the rebel-held city of Khan Sheikhoun on April 4, 2017. Dozens of people were killed, according to multiple activist groups. The United States responded a few days later by launching between 50-60 Tomahawk missiles at a Syrian government airbase. US officials said the base was home to warplanes that carried out the chemical attack. Syria has repeatedly denied it had anything to do with the attack.

Arabic writing that reads "some day we will return" is seen on a bus window as civilians evacuate Aleppo on December 15, 2016. The evacuations began under a new ceasefire between rebels and pro-government forces.

Arabic writing that reads "some day we will return" is seen on a bus window as civilians evacuate Aleppo on December 15, 2016. The evacuations began under a new ceasefire between rebels and pro-government forces.

Smoke rises after an airstrike in Aleppo on October 4, 2016.

Smoke rises after an airstrike in Aleppo on October 4, 2016.

This still image, taken from a video posted by the Aleppo Media Center, shows a young boy in an ambulance after an airstrike in Aleppo, Syria, on August 17, 2016. It took nearly an hour to dig the boy, identified as Omran Daqneesh, out from the rubble, an activist told CNN. The airstrike destroyed his home, where he lived with his parents and two siblings. Director of the Aleppo Media Center Yousef Saddiq said Omran's 10-year-old brother, Ali, died from his injuries.

This still image, taken from a video posted by the Aleppo Media Center, shows a young boy in an ambulance after an airstrike in Aleppo, Syria, on August 17, 2016. It took nearly an hour to dig the boy, identified as Omran Daqneesh, out from the rubble, an activist told CNN. The airstrike destroyed his home, where he lived with his parents and two siblings. Director of the Aleppo Media Center Yousef Saddiq said Omran's 10-year-old brother, Ali, died from his injuries.

Members of a Syrian opposition group attack the headquarters of al-Assad regime forces in the Aleppo villages of Nubul and al-Zahraa on February 12, 2016.

Members of a Syrian opposition group attack the headquarters of al-Assad regime forces in the Aleppo villages of Nubul and al-Zahraa on February 12, 2016.

A sandstorm blows over damaged buildings in the rebel-held area of Douma, east of Damascus, on September 7, 2015.

A sandstorm blows over damaged buildings in the rebel-held area of Douma, east of Damascus, on September 7, 2015.

A refugee carries mattresses as he re-enters Syria from Turkey on June 22, 2015, after Kurdish People's Protection Units regained control of the area around Tal Abyad, Syria, from ISIS.

A refugee carries mattresses as he re-enters Syria from Turkey on June 22, 2015, after Kurdish People's Protection Units regained control of the area around Tal Abyad, Syria, from ISIS.

A Syrian child fleeing the war gets lifted over fences to enter Turkish territory illegally near a border crossing at Akcakale, Turkey, on June 14, 2015.

A Syrian child fleeing the war gets lifted over fences to enter Turkish territory illegally near a border crossing at Akcakale, Turkey, on June 14, 2015.

Nusra Front fighters inspect a helicopter belonging to pro-government forces after it crashed in the rebel-held Idlib countryside on March 22, 2015.

Nusra Front fighters inspect a helicopter belonging to pro-government forces after it crashed in the rebel-held Idlib countryside on March 22, 2015.

Rebel fighters dig caves in the mountains for bomb shelters in the northern countryside of Hama on March 9, 2015.

Rebel fighters dig caves in the mountains for bomb shelters in the northern countryside of Hama on March 9, 2015.

A long-exposure photograph shows a rocket being launched in Aleppo on October 5, 2014.

A long-exposure photograph shows a rocket being launched in Aleppo on October 5, 2014.

Medics tend to a man's injuries at a field hospital in Douma after airstrikes on September 20, 2014.

Medics tend to a man's injuries at a field hospital in Douma after airstrikes on September 20, 2014.

Volunteers remove a dead body from under debris after shelling in Aleppo on August 29, 2014. According to the Syrian Civil Defense, barrel bombs are now the greatest killer of civilians in many parts of Syria. The White Helmets are a humanitarian organization that tries to save lives and offer relief.

Volunteers remove a dead body from under debris after shelling in Aleppo on August 29, 2014. According to the Syrian Civil Defense, barrel bombs are now the greatest killer of civilians in many parts of Syria. The White Helmets are a humanitarian organization that tries to save lives and offer relief.

Photographs of victims of the Assad regime are displayed as a Syrian army defector known as "Caesar," center, appears in disguise to speak before the House Foreign Affairs Committee in Washington. The July 31, 2014, briefing was called "Assad's Killing Machine Exposed: Implications for U.S. Policy." Caesar, apparently a witness to the regime's brutality, smuggled more than 50,000 photographs depicting the torture and execution of more than 10,000 dissidents. CNN cannot independently confirm the authenticity of the photos, documents and testimony referenced in the report.

Photographs of victims of the Assad regime are displayed as a Syrian army defector known as "Caesar," center, appears in disguise to speak before the House Foreign Affairs Committee in Washington. The July 31, 2014, briefing was called "Assad's Killing Machine Exposed: Implications for U.S. Policy." Caesar, apparently a witness to the regime's brutality, smuggled more than 50,000 photographs depicting the torture and execution of more than 10,000 dissidents. CNN cannot independently confirm the authenticity of the photos, documents and testimony referenced in the report.

Rebel fighters execute two men on July 25, 2014, in Binnish, Syria. The men were reportedly charged by an Islamic religious court with detonating several car bombs.

Rebel fighters execute two men on July 25, 2014, in Binnish, Syria. The men were reportedly charged by an Islamic religious court with detonating several car bombs.

A giant poster of al-Assad is seen in Damascus on May 31, 2014, ahead of the country's presidential elections. He received 88.7% of the vote in the country's first election after the civil war broke out.

A giant poster of al-Assad is seen in Damascus on May 31, 2014, ahead of the country's presidential elections. He received 88.7% of the vote in the country's first election after the civil war broke out.

A Free Syrian Army fighter fires a rocket-propelled grenade during heavy clashes in Aleppo on April 27, 2014.

A Free Syrian Army fighter fires a rocket-propelled grenade during heavy clashes in Aleppo on April 27, 2014.

A US ship staff member wears personal protective equipment at a naval airbase in Rota, Spain, on April 10, 2014. A former container vessel was fitted out with at least $10 million of gear to let it take on about 560 metric tons of Syria's most dangerous chemical agents and sail them out to sea, officials said.

A US ship staff member wears personal protective equipment at a naval airbase in Rota, Spain, on April 10, 2014. A former container vessel was fitted out with at least $10 million of gear to let it take on about 560 metric tons of Syria's most dangerous chemical agents and sail them out to sea, officials said.

A man holds a baby who was rescued from rubble after an airstrike in Aleppo on February 14, 2014.

A man holds a baby who was rescued from rubble after an airstrike in Aleppo on February 14, 2014.

An injured man is helped following an airstrike in Aleppo's Maadi neighborhood on December 17, 2013.

An injured man is helped following an airstrike in Aleppo's Maadi neighborhood on December 17, 2013.

Syrian children wait as doctors perform medical checkups at a refugee center in Sofia, Bulgaria, on October 26, 2013.

Syrian children wait as doctors perform medical checkups at a refugee center in Sofia, Bulgaria, on October 26, 2013.

Residents run from a fire at a gasoline and oil shop in Aleppo's Bustan Al-Qasr neighborhood on October 20, 2013. Witnesses said the fire was caused by a bullet from a pro-government sniper.

Residents run from a fire at a gasoline and oil shop in Aleppo's Bustan Al-Qasr neighborhood on October 20, 2013. Witnesses said the fire was caused by a bullet from a pro-government sniper.

The UN Security Council passes a resolution September 27, 2013, requiring Syria to eliminate its arsenal of chemical weapons. Al-Assad said he would abide by the resolution.

The UN Security Council passes a resolution September 27, 2013, requiring Syria to eliminate its arsenal of chemical weapons. Al-Assad said he would abide by the resolution.

An aerial view shows the Zaatari refugee camp near the Jordanian city of Mafraq on July 18, 2013.

An aerial view shows the Zaatari refugee camp near the Jordanian city of Mafraq on July 18, 2013.

Rebels launch a missile near the Abu Baker brigade in Al-Bab, Syria, on January 16, 2013.

Rebels launch a missile near the Abu Baker brigade in Al-Bab, Syria, on January 16, 2013.

Syrians look for survivors amid the rubble of a building targeted by a missile in the al-Mashhad neighborhood of Aleppo on January 7, 2013.

Syrians look for survivors amid the rubble of a building targeted by a missile in the al-Mashhad neighborhood of Aleppo on January 7, 2013.

A father reacts after the deaths of two of his children in Aleppo on January 3, 2013.

A father reacts after the deaths of two of his children in Aleppo on January 3, 2013.

The bodies of three children are laid out for identification by family members at a makeshift hospital in Aleppo on December 2, 2012. The children were allegedly killed in a mortar shell attack that landed close to a bakery in the city.

The bodies of three children are laid out for identification by family members at a makeshift hospital in Aleppo on December 2, 2012. The children were allegedly killed in a mortar shell attack that landed close to a bakery in the city.

Smoke rises in the Hanano and Bustan al-Basha districts in Aleppo as fighting continues through the night on December 1, 2012.

Smoke rises in the Hanano and Bustan al-Basha districts in Aleppo as fighting continues through the night on December 1, 2012.

An Israeli tank crew sits on the Golan Heights overlooking the Syrian village of Breqa on November 6, 2012. Israel fired warning shots toward Syria after a mortar shell hit an Israeli military post. It was the first time Israel fired on Syria across the Golan Heights since the 1973 Yom Kippur War.

An Israeli tank crew sits on the Golan Heights overlooking the Syrian village of Breqa on November 6, 2012. Israel fired warning shots toward Syria after a mortar shell hit an Israeli military post. It was the first time Israel fired on Syria across the Golan Heights since the 1973 Yom Kippur War.

A Syrian rebel walks inside a burnt section of the Umayyad Mosque in Aleppo hours before the Syrian army retook control of the complex on October 14, 2012.

A Syrian rebel walks inside a burnt section of the Umayyad Mosque in Aleppo hours before the Syrian army retook control of the complex on October 14, 2012.

Smoke rises over the streets after a mortar bomb from Syria landed in the Turkish border village of Akcakale on October 3, 2012. Five people were killed. In response, Turkey fired on Syrian targets and its parliament authorized a resolution giving the government permission to deploy soldiers to foreign countries.

Smoke rises over the streets after a mortar bomb from Syria landed in the Turkish border village of Akcakale on October 3, 2012. Five people were killed. In response, Turkey fired on Syrian targets and its parliament authorized a resolution giving the government permission to deploy soldiers to foreign countries.

Free Syrian Army fighters are reflected in a mirror they use to see a Syrian Army post only 50 meters away in Aleppo on September 16, 2012.

Free Syrian Army fighters are reflected in a mirror they use to see a Syrian Army post only 50 meters away in Aleppo on September 16, 2012.

A Syrian man carrying grocery bags dodges sniper fire in Aleppo as he runs through an alley near a checkpoint manned by the Free Syrian Army on September 14, 2012.

A Syrian man carrying grocery bags dodges sniper fire in Aleppo as he runs through an alley near a checkpoint manned by the Free Syrian Army on September 14, 2012.

Family members mourn the deaths of their relatives in front of a field hospital in Aleppo on August 21, 2012.

Family members mourn the deaths of their relatives in front of a field hospital in Aleppo on August 21, 2012.

A Free Syrian Army fighter runs for cover as a Syrian Army tank shell hits a building across the street during clashes in the Salaheddine neighborhood of central Aleppo on August 17, 2012.

A Free Syrian Army fighter runs for cover as a Syrian Army tank shell hits a building across the street during clashes in the Salaheddine neighborhood of central Aleppo on August 17, 2012.

Rebel fighters with the Free Syrian Army capture a police officer in Aleppo, Syria, who they believed to be pro-regime militiaman on July 31, 2012. Dozens of officers were reportedly killed as rebels seized police stations in the city.

Rebel fighters with the Free Syrian Army capture a police officer in Aleppo, Syria, who they believed to be pro-regime militiaman on July 31, 2012. Dozens of officers were reportedly killed as rebels seized police stations in the city.

Syrian refugees walk across a field in Syria before crossing into Turkey on March 14, 2012.

Syrian refugees walk across a field in Syria before crossing into Turkey on March 14, 2012.

Supporters of al-Assad celebrate during a referendum vote in Damascus on February 26, 2012. Opposition activists reported at least 55 deaths across the country as Syrians headed to the polls. Analysts and protesters widely described the constitutional referendum as a farce. "Essentially, what (al-Assad's) done here is put a piece of paper that he controls to a vote that he controls so that he can try and maintain control," a US State Department spokeswoman said.

Supporters of al-Assad celebrate during a referendum vote in Damascus on February 26, 2012. Opposition activists reported at least 55 deaths across the country as Syrians headed to the polls. Analysts and protesters widely described the constitutional referendum as a farce. "Essentially, what (al-Assad's) done here is put a piece of paper that he controls to a vote that he controls so that he can try and maintain control," a US State Department spokeswoman said.

Delegates from Arab League member states and Turkey discuss a response to the government's crackdown in Syria on November 16, 2011.

Delegates from Arab League member states and Turkey discuss a response to the government's crackdown in Syria on November 16, 2011.

Jamal al-Wadi speaks in Istanbul on September 15, 2011, after an alignment of Syrian opposition leaders announced the creation of a Syrian National Council -- their bid to present a united front against Bashar al-Assad's regime and establish a democratic system.

Jamal al-Wadi speaks in Istanbul on September 15, 2011, after an alignment of Syrian opposition leaders announced the creation of a Syrian National Council -- their bid to present a united front against Bashar al-Assad's regime and establish a democratic system.

Syrian children walk over bricks stored for road repairs during a spontaneous protest June 15, 2011, at a refugee camp near the Syrian border in Yayladagi, Turkey.

Syrian children walk over bricks stored for road repairs during a spontaneous protest June 15, 2011, at a refugee camp near the Syrian border in Yayladagi, Turkey.

Anti-government protesters demonstrate in Daraa on March 23, 2011. In response to continuing protests, the Syrian government announced several plans to appease citizens.

Anti-government protesters demonstrate in Daraa on March 23, 2011. In response to continuing protests, the Syrian government announced several plans to appease citizens.

An injured man lying in the back of a vehicle is rushed to a hospital in Daraa, Syria, on March 23, 2011. Violence flared in Daraa after a group of teens and children were arrested for writing political graffiti. Dozens of people were killed when security forces cracked down on demonstrations.

An injured man lying in the back of a vehicle is rushed to a hospital in Daraa, Syria, on March 23, 2011. Violence flared in Daraa after a group of teens and children were arrested for writing political graffiti. Dozens of people were killed when security forces cracked down on demonstrations.

Displaced Syrian residents wait to receive food aid distributed by the UN Relief and Works Agency at the besieged al-Yarmouk camp, south of Damascus, Syria, on January 31, 2014. According to the UN Envoy for Syria, an estimated 400,000 Syrians have been killed since an uprising in March 2011 spiraled into civil war. See how the conflict has unfolded.

Displaced Syrian residents wait to receive food aid distributed by the UN Relief and Works Agency at the besieged al-Yarmouk camp, south of Damascus, Syria, on January 31, 2014. According to the UN Envoy for Syria, an estimated 400,000 Syrians have been killed since an uprising in March 2011 spiraled into civil war. See how the conflict has unfolded.

As Arab Spring demonstrations overthrow governments across the Middle East, a group of children in Daraa, southern Syria, are arrested and allegedly tortured for scrawling graffiti on a school reading "the people want to topple the regime."

Outrage over the arrests turns into protests and a brutal crackdown

Protesters clash with Syrian security forces on the streets of Daraa in March 2011.

March 2011: Syrians protest in cities across the country, demanding the government enact reforms and release political protesters. Government forces fire on the demonstrations, Syrians protest in cities across the country, demanding the government enact reforms and release political protesters. Government forces fire on the demonstrations, killing dozens.

As the bloodshed gets worse, the opposition gets organized

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July 2011: As the government crackdown intensifies, a group of Syrian military officers As the government crackdown intensifies, a group of Syrian military officers defect and form the Free Syrian Army . In a video posted on YouTube, they say thousands of soldiers have left their posts instead of firing on protesters, and they promise to wage guerrilla war against regime forces. Three months later, a handful of political opposition groups establish the Syrian National Council , aimed at toppling President Bashar al-Assad

The West calls for Assad to quit, but Russia and China have other ideas

Meanwhile, the influence of Islamist groups grows in rebel ranks

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The U.N. brokers a ceasefire that falls apart almost immediately

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April 2012: Former United Nations chief Kofi Annan Former United Nations chief Kofi Annan brokers a ceasefire plan calling for a halt to violence and the implementation of a political process to end the crisis, but the plan never gets off the ground.

As refugees flee, the U.N. accuses Syria of crimes against humanity

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June to July 2012: A top U.N. human rights official accuses Syria of A top U.N. human rights official accuses Syria of engaging in crimes against humanity. A month later, the Zaatari refugee camp opens in Jordan to house the thousands of refugees streaming across the border from Syria.

Barack Obama announces his 'red line' in Syria for the first (but not last) time

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August 2012: President Obama says President Assad President Obama says President Assad will be crossing a "red line" if he uses chemical weapons against his own people in Syria. Syria has one of the largest and most advanced chemical warfare programs in the Arab world, according to experts.

Two years in, 60,000 are dead, and the U.S. says it will send aid to rebels

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February 2013: The U.S. The U.S. promises to send food and medical supplies -- but not weapons -- to Syrian rebels. It's the first such move since the conflict began two years before, in an effort to hem in the radical Islamist groups vying for influence in Syria. More than 60,000 people are now dead and nearly a million have fled the country.

ISIS emerges in Syria

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Spring 2013: The The Islamic State in Iraq and Syria (ISIS) announces its arrival in northern Syria by seizing the city of Raqqa. The group began in 2004 as al Qaeda in Iraq before rebranding as ISIS two years later. The aim of ISIS -- led by Abu Bakr al-Baghdadi -- is to impose its strict Islamist ideology on Syrians in the areas it controls.

European nations join the list of countries sending weapons into Syria

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Syria crosses Obama's 'red line,' and the U.S. prepares to attack

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August 2013: Hundreds of people are killed in Hundreds of people are killed in a suspected chemical weapons attack in rebel-held areas on the outskirts of Damascus. Obama asks Congress to authorize military action against Syria.

John Kerry goes off script, and the Russians pounce

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September 2013: U.S. airstrikes seem inevitable. Asked what, if anything, the Syrian regime can do to stop an attack, U.S. airstrikes seem inevitable. Asked what, if anything, the Syrian regime can do to stop an attack, U.S. Secretary of State John Kerry says Assad "could turn over every single bit of his chemical weapons to the international community in the next week." Russia immediately proposes putting Syria's chemical weapons under international control, a plan that staves off U.S. military action in the country for the time being.

140,000 are dead in Syria as peace talks go nowhere fast

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February 2014: A second round of peace talks between the Syrian government, the opposition and an array of world powers A second round of peace talks between the Syrian government, the opposition and an array of world powers ends without a solution. At least 140,000 Syrians are now dead, opposition groups say, and hundreds of thousands have been displaced.

ISIS declares an independent state in Iraq and Syria

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June 2014: ISIS ISIS announces the establishment of a caliphate (Islamic state) stretching from western Syria to eastern Iraq. The group rebrands itself Islamic State and says Abu Bakr al-Baghdadi is the leader of this new state. The declaration comes after ISIS seizes Mosul , Iraq's second-biggest city.

ISIS beheads American journalist James Foley

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The U.S. launches airstrikes on ISIS in Syria

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September 2014: American jets American jets begin bombing ISIS targets in Syria, including the group's stronghold in Raqqa. The foreign partners participating in the strikes are Arab countries, including Saudi Arabia, Jordan, Bahrain and the United Arab Emirates.

Kurdish forces, backed by U.S. strikes, take back Kobani from ISIS

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January 2015: Kurdish fighters retake Kobani , a key Syrian town on the Turkish border, after nearly four months of fighting with ISIS.

ISIS seizes Palmyra and blows up its priceless ruins

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Russia launches airstrikes in Syria

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September 30, 2015: After weeks of bulking up its military presence in Syria, After weeks of bulking up its military presence in Syria, Russia launches airstrikes on rebel targets in the country for the first time. Russia's intervention will prove decisive in swinging momentum in the conflict the Syrian regime's way.

Turkey downs Russian jet over Syria

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November 24, 2015: Turkey Turkey shoots down a Russian Su-24 warplane over Syria , saying it had violated Turkish airspace. The incident triggers a war of words and escalating tensions between the two powers, who back opposite sides in the Syrian conflict.

Europe hits 1 million refugees in 2015

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December, 2015: The The number of migrants who have entered Europe by sea and land in 2015 passes 1 million , the International Organization for Migration says, with about half of them Syrians fleeing the conflict. The surge in migrants poses a crisis for European nations, with governments split on how to respond.

Donors pledge more than $10 billion to Syria

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February 4, 2016: More than More than $10 billion is pledged for Syria by international donors at a conference in London. But the goodwill is overshadowed a brutal Syrian government offensive, backed by Russian air power, on the northern city of Aleppo, which Turkey says has sent a fresh wave of refugees fleeing toward its border.

World powers agree to 'cessation of hostilities'

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