Iran

What it's done: Tehran has been one of Assad's strongest backers, supporting loyalist forces with money, weapons and intelligence. Iran has also sent military advisers from its Islamic Revolutionary Guard Corps to Syria and directed fighters from Hezbollah, a Lebanon-based militant group backed by Iran, which is also involved in the conflict. It has also organized paramilitary militia from Syria, Afghanistan and Iraq to fight for the Assad regime. Iran has set up multiple bases in Syria, raising concern in Israel that Tehran plans to stay in Syria for the longhaul.

Why it's there: Iran and Syria had a mutual defense pact before the onset of the Syria civil war in 2011. Iran has sought to bolster the Assad regime in its fight against various rebel factions, many of which are backed Tehran's regional rivals such as Saudi Arabia and the Gulf States. Iran, which also backs Shiite militia in Iraq fighting the "Islamic State," views the Syrian war as a frontline against Sunni militant groups and as a means to expand its regional influence. The Assad regime allows Iranian aid to flow to Hezbollah, a major regional military power and enemy of Israel.

Russia

What it's done: Moscow came to Assad's aid in late 2015, deploying hundreds of troops and acting as the regime's air force. Russian officials say it targets terrorist organizations like "Islamic State" (IS). But Russian bombers have also struck other anti-Assad groups, turning the tide of the war in the regime's favor. As a veto-wielding UN Security Council member, Russia has also provided the Syrian regime diplomatic cover. Alongside Iran and Turkey, Moscow has sought to find a political end to the conflict parallel to UN talks in Geneva.

Why it's there: Moscow wants to secure its influence in the Middle East by keeping Assad in office and maintaining an important military air base in the western province of Latakia and a naval base in the port city of Tartus. Russian President Vladimir Putin also appears to want to bolster Russian prestige and influence in the Middle East at the expense of the United States, which it blames for creating instability.

Saudi Arabia

What it's done: Riyadh has given money and weapons to Syrian opposition forces, including some Islamist militant groups such as the Army of Islam. It has also played a limited role in US-led international coalition against IS.

Why it's there: Saudi Arabia, a majority Sunni country, opposes Iran's attempts to expand its influence in the Middle East. Riyadh wants to replace Assad with a pro-Saudi, anti-Iranian leader.

Read more: Opinion: The twisted logic of the war in Syria

How the Syrian civil war began - and gave 'Islamic State' room to grow The "Arab Spring" effect In 2011, as regimes crumble across the region, tens of thousands of Syrians take to the streets to protest against corruption, high unemployment and soaring food prices. The Syrian government responds with live ammunition, claiming some 400 lives by May.

How the Syrian civil war began - and gave 'Islamic State' room to grow Condemnation without consensus At the urging of Western countries, the UN Security Council condemns the violent crackdown. The EU and US implement an arms embargo, visa bans and asset freezes in the months that follow. With the backing of the Arab League, calls eventually grow for the Syrian president's departure. But not all UN members agree with this demand.

How the Syrian civil war began - and gave 'Islamic State' room to grow Assad refuses to back down Bashar al-Assad - who has been in power since the death of his father in 2000 - sees his reputation wane with the continuing unrest. He refuses to end decades-long emergency rule, which allows for surveillance and interrogation. Russia backs its ally, supplying weapons and vetoing UN resolutions on Syria multiple times.

How the Syrian civil war began - and gave 'Islamic State' room to grow The opposition gets organized By the end of the year, human rights groups and the UN have evidence of human rights abuses. Civilians and military deserters are slowly beginning to organize themselves to fight back against government forces, which have been targeting dissidents. More than 5,000 have died so far in the fighting. It will take another six months before the UN acknowledges that a war is taking place on Syrian soil.

How the Syrian civil war began - and gave 'Islamic State' room to grow Outside intervention In September 2012, Iran confirms that it has fighters on the ground in Syria - a fact long denied by Damascus. The presence of allied troops underscores the hesitance of the US and other Western powers to intervene in the conflict. The US, stung by failed interventions in Afghanistan and Iraq, sees dialogue as the only reasonable solution.

How the Syrian civil war began - and gave 'Islamic State' room to grow Fleeing the conflict As the death toll nears 100,000, the number of refugees in neighboring countries - such as Turkey and Jordan - hits one million. That number will double by September 2013. The West and the Arab League have seen all attempts at a transitional government fail in the two years of war, watching as fighting spills over into Turkey and Lebanon. They fear Assad will stay in power by any means possible.

How the Syrian civil war began - and gave 'Islamic State' room to grow No united front against Assad Assad has long claimed he's combatting terrorists. But it's not until the second year of war that the fragmented Free Syrian Army is definitely known to include radical extremists. The group Al-Nusra Front pledges allegiance to al Qaeda, further splintering the opposition.

How the Syrian civil war began - and gave 'Islamic State' room to grow From brute force to chemical warfare In June 2013, the White House says it has evidence that Assad has been using sarin nerve gas on civilians - a report later backed by the UN. The discovery pushes US President Barack Obama and other Western leaders toward considering the use of military force. However, Russia's proposal to remove the chemical weapons ultimately wins out.

How the Syrian civil war began - and gave 'Islamic State' room to grow Islamic State emerges Reports of a new jihadist group calling itself the Islamic State of Iraq and the Levant (ISIL) emerge in the final weeks of 2013. Taking land in northern Syria and Iraq, the group sparks infighting among the opposition, with some 500 dead by early 2014. The unexpected emergence of IS ultimately draws the US, France, Saudi Arabia and other nations into the war. Author: Kathleen Schuster



Turkey

What it's done: Turkey had a good relationship with Syria in the mid-2000s. Since the outbreak of the civil war, Turkey has supported non-Kurdish Syrian opposition groups seeking to topple Assad. Turkey has allowed opposition fighters, including jihadist militants, to enter the fray across the Turkish-Syrian border. In addition to fighting IS, the Turkish military and its rebel allies have conducted two operations in northern Syria against the Kurds and carved out a zone of influence.

Why it's there: Turkey originally sought to topple the Assad regime by backing various rebel groups. After Russia intervened in Syria in 2015, Turkey has focused more on preventing Syrian Kurds from gaining autonomy in northern Syria. Ankara fears that Kurdish gains could embolden the Kurdistan Workers' Party (PKK), which has fought a more than three decade insurgency against Turkey. Ankara considers the US-backed Syrian Kurds as a terrorist group tied to the PKK. Some 3 million Syrian refugees are in Turkey and Ankara seeks to prevent new refugee flows and carve out safe zones in northern Syria to house refugees.

Read more: Turkey's military offensive against Kurdish-held Afrin: What you need to know

Israel

What it's done: Israel has launched airstrikes against the Lebanese Shiite militia Hezbollah and Iranian targets in Syria, especially suspected weapons shipments and bases. It has also backed several smaller rebel groups and Druze along the border in the Golan Heights to create a buffer zone.

Why it's there:

Israel wants to prevent Iran from developing a political and military presence on its northern doorstep. Israel also wants to rollback Hezbollah in Syria in order to prevent it from forming a wider northern front, alongside its main backer Iran.

Hezbollah and Israel last fought a war in 2006, but since then the group has become stronger. Israeli intelligence estimates Hezbollah has more 100,000 missiles stockpiled, including advance missiles provided by Iran, that in the event of a war with Israel would overrun its missile defense system and be able to strike cities as far as southern Israel.

Read more: Former US ambassador: 'Syrian situation extremely dangerous'

What is the 'Islamic State'? Where did it come from? The "Islamic State" (IS) — also known as ISIL, ISIS and Daesh — is an al-Qaida splinter group with a militant Sunni Islamist ideology. It emerged in the aftermath of the US-led invasion of Iraq in 2003 and is led by Abu Bakr al-Baghdadi. Their goal is to create a worldwide "caliphate." It gained worldwide notoriety in 2014 after a blitzkrieg military campaign that resulted in the capture of Mosul.

What is the 'Islamic State'? Where does it operate? IS is believed to be operational in more than a dozen countries across the world. It controls territories in Iraq and Syria. However, the group has lost much of the territory it controlled in Iraq and Syria at the height of its expansion in 2014.

What is the 'Islamic State'? Who is fighting back? The US leads an international coalition of more than 50 countries, including several Arab nations. Russia, Iran and its Lebanese Shiite ally Hezbollah, which all support the Syrian government, also fight IS. Regional forces such as the Kurdish peshmerga (above) and US-backed Syrian Kurdish fighters, fight IS on the ground. The Iraqi army and militia have pushed IS from large parts of the country.

What is the 'Islamic State'? How does it fund itself? One of IS' main sources of income has been oil and gas. At one point, it controlled an estimated one-third of Syria's oil production. However, US-led airstrikes deliberately targeted oil resources and the Syrian government as well as US-backed Syrian Kurdish fighters have retaken most oil wells. Other means of income include taxes, ransom, selling looted antiquities and extortion.

What is the 'Islamic State'? Where does it carry out attacks? IS has claimed responsibility for numerous terrorist attacks across the globe. The militant group has targeted capitals across the EU, including Berlin, Brussels and Paris. IS leaders have encouraged so-called "lone wolf" attacks, whereby individuals who support IS carry out terrorist acts without the direct involvement of the group.

What is the 'Islamic State'? What other tactics does it use? The group uses various tactics to expand its power. IS fighters have looted and destroyed historical artifacts in Syria and Iraq in an attempt at "cultural cleansing." The group has also enslaved thousands of women from religious minority groups, including Yazidis. IS also uses a sophisticated social network to distribute propaganda and recruit sympathizers.

What is the 'Islamic State'? How has it impacted the region? IS has further exacerbated the ongoing Syrian conflict. Millions of Syrians and Iraqis have fled their homes, many traveling to Europe in pursuit of refuge. Although it has lost all of its strongholds, the militant group has left extraordinary destruction in its wake. Areas affected by the militant group's rule will likely take years to rebuild. Author: Rachel Stewart



United States

What it's done: The US has led an international coalition fighting IS with airstrikes and special forces since 2014. It has also provided air support and weapons to opposition groups in northern Syria, including Kurdish forces opposed to Turkey, a US ally in NATO. Washington has also deployed about 2000 special forces to fight alongside the Syrian Democratic Forces (SDF), a mixed Kurdish and Arab force in control of about 25 percent of Syrian territory.

Why it's there: Washington's foremost stated goal has been the destruction of IS and other extremist groups in Syria. US policy toward Assad is less clear. Trump's predecessor, Barack Obama, said "Assad must go." Apart from its opposition to the Syrian government's use of chemical weapons, the Trump administration's position on Assad's future is more ambiguous. The US also seeks to rollback Iranian influence in Syria.

Read more: How German Tornado jets can help the anti-IS alliance in Syria

Germany

What it's done: Germany has flown surveillance flights over Syrian territory to support airstrikes against IS. Berlin has also called on Russia and Iran to persuade Assad to leave office in any peace deal. Germany is a major funder of humanitarian aid to Syrian refugees.

Why it's there: Berlin also wants to see the defeat of IS, which has carried out terror attacks in Europe. It has also opposed the Assad regime. German officials have said there can be no lasting peace in Syria if Assad remains in power.

NATO partners adrift: USA and Turkey Jovial gestures belie multiple disputes May 16, 2017: Trump welcomes Erdogan to Washington, saying both presidents have a "great relationship" and would make it "even better." Erdogan congratulates Trump on his "legendary" 2016 election win but complains bitterly about US arming of the Kurdish YPG militia, claiming that its inclusion in the US-led campaign against IS in in war-torn Syria provides a cover for Kurdish separatism.

NATO partners adrift: USA and Turkey Melee becomes further irritant May 17: As Erdogan ends his visit, Voice of America video footage emerges showing his guards assaulting Kurdish protesters outside the Turkish ambassador's residence in Washington. A month later, US authorities issue arrest warrants for 12 members of Erdogan's security detail, who had long returned to Turkey. US Secretary of State Rex Tillerson says the assaults breached "legitimate" free speech.

NATO partners adrift: USA and Turkey First anniversary of coup attempt July 15, 2017: Turkey marks the first anniversary of the failed coup attempt. In a post-coup bid crackdown 50,000 people were arrested, accused of links to the US-based cleric Fethullah Gulen, an Erdogan ally-turned-rival. Tens of thousands more face job suspensions. The refusal of the US to extradite Gulen has been a major sore spot in relations.

NATO partners adrift: USA and Turkey Turkey 'uneasy' about US arming of Kurdish militia August 23: US Defense Secretary James Mattis visits Ankara as the Pentagon stresses US commitment to bilateral relations and "honest dialogue." Mattis had just visited Iraq to assess the anti-IS campaign. Erdogan tells Turkish media that Turkey will thwart any attempt by the Kurdish People's Protection Units (YPG) to establish a "terror corridor" in northern Syria through to the Mediterranean.

NATO partners adrift: USA and Turkey Turkey arrests US consulate employee October 5: Turkish authorities arrest Metin Topuz, a Turkish national employed at the US consulate in Istanbul. He is formally charged with espionage and collaboration in the 2016 coup attempt. The US embassy in Ankara subsequently says it is "deeply disturbed" by the arrest. It's reportedly the second since March, when a Turkish US consulate employee was arrested in Adana.

NATO partners adrift: USA and Turkey US and Turkey suspend their respective visa services October 8-9: The United States suspends its issuance of non-immigrant visa applications to Turkish nationals, saying it has to "reassess" Turkish readiness to respect security at US diplomatic missions. Turkey suspends its visa services for US nationals and summons another staffer at the US consulate in Istanbul.

NATO partners adrift: USA and Turkey Attempts to make amends November 6: The US Embassy in Ankara announces that it is reinstating its visa program for Turkish tourists on a "limited" basis after receiving assurances from the government that no employees will be detained "for carrying out official duties." Shortly thereafter, Turkey confirms that it is also resuming visa services for US citizens one day before Prime Minister Yildirim visits Washington.

NATO partners adrift: USA and Turkey At odds over Russian missiles December through August, 2018: In December, Turkey announced it would buy the Russian S-400 missile system, which is incompatable with NATO systems. The US Congress has included a provision in a defense bill that would cut Turkey out of the F-35 fighter jet program if it moves forward with the S-400 deal.

NATO partners adrift: USA and Turkey Release the pastor ... or else August 1, 2018: The US sanctions Turkey's interior and justice ministers over the continued detention of pastor Andrew Brunson. Brunson had been moved from prison to house arrest in late July, but that fell short of US demands for his immediate release and end to terror and espionage charges. Brunson was arrested almost two years ago.



France

What it's done: France initially sent medical supplies and weapons to opposition forces. In 2015, it began airstrikes against IS that intensified after an IS terror attack in Paris in November 2015. French special forces are deployed on the ground alongside US special forces backing the SDF. Paris has also warned Assad against using chemical weapons.

Why it's there: Paris wants to defeat IS after a string of IS-related terrorist attacks in France. French President Emmanuel Macron said in 2017 his country would no longer condition peace talks on a promise by Assad to leave office.



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