April 29, 2011: U.S. sanctions Syria’s intelligence agency for Syria’s crackdown on “Arab Spring” anti-government protests.
Aug. 28, 2011: U.S. President Barack Obama calls for Syrian President Bashar al-Assad to leave office.
Aug. 20, 2012: Obama warns Assad against using chemical weapons in the conflict.
April 2013: Abu Bakr al-Baghdadi announces the formation of the Islamic State in Iraq and the Levant (IS), an Islamic militant group.
March 2013: Syria bombs the rebel-held northern city of Raqqa. The U.S. and Britain promise nonlethal support for moderate rebels.
Aug. 30, 2013: The U.S. condemns the Syrian government for reportedly gassing roughly hundreds of “women and children and innocent civilians.”
Aug. 31, 2013: Obama seeks congressional authorization to use military force against Syria.
September 2013: UN weapons inspectors verify chemical weapons killed roughly 300 people in Damascus.
December 2013: After IS fighters seize warehouses used by the Western-backed Free Syrian Army rebels, U.S. suspends aid.
June 2014: IS militants seize Mosul and Tikrit in Iraq and announce plans to establish a caliphate.
August 8, 2014: Obama authorizes “targeted airstrikes,” with two rounds of attacks hitting IS targets in northwestern Iraq.
Aug. 20, 2014: IS releases a video showing the beheading of American journalist James Foley, 40, kidnapped a year earlier in northern Syria. His executioner warns the U.S. government that trying “to deny the Muslims their rights of living in safety under the Islamic caliphate will result in the bloodshed of your people.”
Sept. 2, 2014: IS releases a video showing the decapitation of American journalist Steven Sotloff, kidnapped in August 2013 near Aleppo, Syria.
Sept. 22, 2014: A U.S.-led coalition – including Bahrain, Jordan, Qatar, Saudi Arabia and the United Arab Emirates – begins a campaign of airstrikes against IS targets in Syria.
Feb. 10, 2015: U.S. officials confirm the death of American aid worker Kayla Mueller, 26. Kidnapped in Aleppo in August 2013, she reportedly had been held by IS senior member Abu Sayyaf in eastern Syria and was repeatedly raped by IS leader Baghdadi. IS claims Mueller died Feb. 6 in a weapons warehouse near Raqqa.
May 16, 2015: U.S. Special Operations forces kill Sayyaf and capture his wife, Umm Sayyaf, in a ground raid in eastern Syria.
May 20, 2015: IS militants seize control of most of Palmyra, a central Syrian city with 2,000-year-old ruins designated a UNESCO World Heritage Site. IS militants subsequently destroy two Roman-era temples.
September 30, 2015: Russia begins airstrikes in support of Syria’s Assad government. Moscow claims the campaign targets IS, but the Pentagon and Western officials note Western-backed rebels are also hit.
Oct. 30, 2015: Obama authorizes fewer than 50 U.S. special operations forces to coordinate movements of Syrian local fighters with the U.S.-led coalition.
Reuters news agency contributed to this report.