Russian troops on patrol in the eastern Syrian city of Deir el-Zour in September. Moscow's involvement in the conflict changed its course but is steeped in controversy.

A Russian navy captain looks over the missile cruiser Moskva, on patrol in the Mediterranean Sea, on Dec. 17, 2015. Fighting a conflict far from Russian territory relies on Moscow's ability to project military force over considerable distances. Without a land route option, Russia relied on airlifts and its maritime transport capacity to move troops to Syria and sustain them.

A Syrian man wields portraits of President Bashar al Assad and his Russian counterpart, Vladimir Putin. For Syrians who support the al Assad administration, Moscow's support has been well-received. For those opposing Damascus, the Russian presence has been punishing. 

A Russian Sukhoi Su-35 fighter lands at the Russian Hmeimim air base in Latakia province, in the northwest of Syria. Moscow has used the conflict to promote its equipment, leading to lucrative foreign arms sales. Su-34 and Su-35 combat aircraft featured prominently, along with the S-400 surface-to-air missile system.

Syrians gather around the rubble of a hospital supported by Doctors Without Borders near Maarat al-Nuaman, in the northern province of Idlib. The building was hit by suspected Russian airstrikes on Feb. 15, 2016.

A pair of Russian S-400 surface-to-air missile launchers at the Hmeimim air base. Moscow has long touted the effectiveness of its advanced air defense systems and their deployment to Syria proved a boon for arms sales. 

A car drives past the wreckage of another vehicle on the outskirts of Deir el-Zour as Syrian government forces press forward with Russian air cover in the September offensive against Islamic State jihadists across the province.

Members of the Russian military police take pictures with displaced Syrian children at a pediatric field hospital in a camp in Jibreen, on the outskirts of Aleppo.

A former rebel fighter returns arms to a representative of the Syrian government accompanied by Russian military personnel in the village of Kawkab, near the city of Hama, on May 4, 2016.

People lay flowers at the Sochi River embankment in Russia the day after a military plane crashed on its way to Syria during late December 2016. Of the 92 onboard, including dozens of Red Army Choir members planning to celebrate the New Year with the troops, none survived.