Syria's civil war has raged on for more than eight years. What began in 2011 as a popular revolt against President Bashar al-Assad spiraled into a bloody humanitarian crisis fueled by the world's biggest players, including the U.S., Russia, Iran, Turkey and the Gulf monarchies.
Russia entered the war in 2015 at the request of Assad's government. Russia had previously supplied weapons and training to Assad's forces, and Russian support allowed the Syrian military to retake territory throughout the country.
Russia has also repeatedly blocked cease-fire resolutions by the United Nations Security Council. Russian officials say the resolutions don't do enough to combat the armed groups Russia considers "terrorists."
But critics of the Russian intervention say the Kremlin is shielding Assad from scrutiny over alleged human rights abuses, like the reported use of chemical weapons. Monitoring groups like Human Rights Watch and Amnesty International have also frequently accused Russia of downplaying the civilian cost of its airstrikes.
This timeline details just some of Russia's operations in Syria since becoming engaged in the war four years ago:
September 2015: Russia launches airstrikes against so-called "Islamic State" targets near Homs province. It was Russia's first major military action beyond the former Soviet bloc since the end of the Cold War.
September 2016: Russia gives air support to Syria's offensive in Aleppo. The campaign reportedly results in the deaths of aid workers and civilians in the area. U.S. and Russian officials agree to coordinate airstrikes against so-called "jihadist" groups, including ISIS. The deal is made, in part, to help bring humanitarian relief to besieged areas.
November 2016: Russia carries out "massive strikes" against targets in Idlib and Homs provinces.
October 2017: Russia vetoes a U.S.-drafted security council resolution to extend an investigation into a chemical weapons attack — its ninth block to date. U.S. officials accuse Russia of protecting the "Assad regime," but Russian Ambassador Vasily Nebenzya says the mandate deserves further review to ensure impartiality.
And recently, an investigation by The New York Times found that the Russian Air Force bombed four hospitals in May over a 12-hour period — a charge dismissed by a Russian foreign ministry spokesman.