Photos: Syria's civil war, in pictures 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. Hide Caption 1 of 52

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Photos: Syria's civil war, in pictures 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. Hide Caption 3 of 52

Photos: Syria's civil war, in pictures Anti-government protesters demonstrate in Daraa on March 23, 2011. In response to continuing protests, the Syrian government announced several plans to appease citizens. Hide Caption 4 of 52

Photos: Syria's civil war, in pictures 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. Hide Caption 5 of 52

Photos: Syria's civil war, in pictures 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. Hide Caption 6 of 52

Photos: Syria's civil war, in pictures Delegates from Arab League member states and Turkey discuss a response to the government's crackdown in Syria on November 16, 2011. Hide Caption 7 of 52

Photos: Syria's civil war, in pictures 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. Hide Caption 8 of 52

Photos: Syria's civil war, in pictures Syrian refugees walk across a field in Syria before crossing into Turkey on March 14, 2012. Hide Caption 9 of 52

Photos: Syria's civil war, in pictures 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. Hide Caption 10 of 52

Photos: Syria's civil war, in pictures 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. Hide Caption 11 of 52

Photos: Syria's civil war, in pictures Family members mourn the deaths of their relatives in front of a field hospital in Aleppo on August 21, 2012. Hide Caption 12 of 52

Photos: Syria's civil war, in pictures 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. Hide Caption 13 of 52

Photos: Syria's civil war, in pictures 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. Hide Caption 14 of 52

Photos: Syria's civil war, in pictures 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. Hide Caption 15 of 52

Photos: Syria's civil war, in pictures 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. Hide Caption 16 of 52

Photos: Syria's civil war, in pictures 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. Hide Caption 17 of 52

Photos: Syria's civil war, in pictures Smoke rises in the Hanano and Bustan al-Basha districts in Aleppo as fighting continues through the night on December 1, 2012. Hide Caption 18 of 52

Photos: Syria's civil war, in pictures 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. Hide Caption 19 of 52

Photos: Syria's civil war, in pictures A father reacts after the deaths of two of his children in Aleppo on January 3, 2013. Hide Caption 20 of 52

Photos: Syria's civil war, in pictures 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. Hide Caption 21 of 52

Photos: Syria's civil war, in pictures Rebels launch a missile near the Abu Baker brigade in Al-Bab, Syria, on January 16, 2013. Hide Caption 22 of 52

Photos: Syria's civil war, in pictures An aerial view shows the Zaatari refugee camp near the Jordanian city of Mafraq on July 18, 2013. Hide Caption 23 of 52

Photos: Syria's civil war, in pictures 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. Hide Caption 24 of 52

Photos: Syria's civil war, in pictures 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. Hide Caption 25 of 52

Photos: Syria's civil war, in pictures Syrian children wait as doctors perform medical checkups at a refugee center in Sofia, Bulgaria, on October 26, 2013. Hide Caption 26 of 52

Photos: Syria's civil war, in pictures An injured man is helped following an airstrike in Aleppo's Maadi neighborhood on December 17, 2013. Hide Caption 27 of 52

Photos: Syria's civil war, in pictures A man holds a baby who was rescued from rubble after an airstrike in Aleppo on February 14, 2014. Hide Caption 28 of 52

Photos: Syria's civil war, in pictures 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. Hide Caption 29 of 52

Photos: Syria's civil war, in pictures A Free Syrian Army fighter fires a rocket-propelled grenade during heavy clashes in Aleppo on April 27, 2014. Hide Caption 30 of 52

Photos: Syria's civil war, in pictures 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. Hide Caption 31 of 52

Photos: Syria's civil war, in pictures 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. Hide Caption 32 of 52

Photos: Syria's civil war, in pictures 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. Hide Caption 33 of 52

Photos: Syria's civil war, in pictures 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. Hide Caption 34 of 52

Photos: Syria's civil war, in pictures Medics tend to a man's injuries at a field hospital in Douma after airstrikes on September 20, 2014. Hide Caption 35 of 52

Photos: Syria's civil war, in pictures A long-exposure photograph shows a rocket being launched in Aleppo on October 5, 2014. Hide Caption 36 of 52

Photos: Syria's civil war, in pictures Rebel fighters dig caves in the mountains for bomb shelters in the northern countryside of Hama on March 9, 2015. Hide Caption 37 of 52

Photos: Syria's civil war, in pictures Nusra Front fighters inspect a helicopter belonging to pro-government forces after it crashed in the rebel-held Idlib countryside on March 22, 2015. Hide Caption 38 of 52

Photos: Syria's civil war, in pictures 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. Hide Caption 39 of 52

Photos: Syria's civil war, in pictures 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. Hide Caption 40 of 52

Photos: Syria's civil war, in pictures A sandstorm blows over damaged buildings in the rebel-held area of Douma, east of Damascus, on September 7, 2015. Hide Caption 41 of 52

Photos: Syria's civil war, in pictures 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. Hide Caption 42 of 52

Photos: Syria's civil war, in pictures 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. Hide Caption 43 of 52

Photos: Syria's civil war, in pictures Smoke rises after an airstrike in Aleppo on October 4, 2016. Hide Caption 44 of 52

Photos: Syria's civil war, in pictures 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. Hide Caption 45 of 52

Photos: Syria's civil war, in pictures 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. Hide Caption 46 of 52

Photos: Syria's civil war, in pictures 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. Hide Caption 47 of 52

Photos: Syria's civil war, in pictures Residents of the war-torn city of Douma break their Ramadan fast on June 18, 2017. Hide Caption 48 of 52

Photos: Syria's civil war, in pictures 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. Hide Caption 49 of 52

Photos: Syria's civil war, in pictures 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. Hide Caption 50 of 52

Photos: Syria's civil war, in pictures 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. Hide Caption 51 of 52