The Islamic State (IS) killed dozens of Russian and Syrian soldiers on Wednesday, May 23, in a surprise attack near the town of Mayadeen in the Deir Az Zor province in eastern Syria. According to a statement released by IS, the group’s fighters also allegedly destroyed several military trucks and armored vehicles and damaged a rocket launcher. The violent clashes come amidst a recent surge of attacks in Syria’s eastern province. Despite recent international criticism that Syrian President Bashar al-Assad used chemical weapons against his citizens in April of this year, Russian support for the Syrian regime shows no sign of waning.

According to U.K.-based organization the Syrian Observatory of Human Rights (SOHR), nine Russian soldiers and at least 26 members of the Syrian government forces died in the attack. SOHR director Rami Abdel Rahman told AFP, “[T]here were 35 pro-government forces killed, including at least nine Russians. Some of those Russian nationals were government troops, but not all of them.” Russian officials claim that 43 militants were killed during the recent hostilities.

A few days later, on Saturday, May 26, IS militants launched a new series of attack in the Deir Az Zor province near Mayadeen. They also targeted Boukamal, a town near the border between Syria and Iraq. SOHR claims that 76 Syrian and Russian troops were killed, compared to 25 IS fighters, though Russia insists the number of IS casualties is significantly higher.

The province of Deir Az Zor was an IS stronghold until November 2017 when the Syrian army recaptured it. Throughout the offensive, which began in the summer of 2017, Russia provided active aerial support for Syrian troops, bombers, attack planes and helicopters. However, IS has resumed attacks in the province, forcing Russian and Syrian pro-regime forces to defend their positions. The most recent clashes broke out on Wednesday when multiple armed groups fired mortar at a Syrian government troop’s artillery battery.

Russia has been the primary ally of the Syrian regime during its ongoing civil war, which began in 2011 when the Assad regime brutally cracked down on Arab Spring protesters. The war has since cost an estimated 400,000 lives and forced more than a million people to flee their homes, according to the United Nations.

According to Russian officials, at least 92 Russian soldiers have died in Syria since September 2015, when Russia began providing aerial and ground support to the Assad regime. Its direct intervention in the war has played a significant role in swaying the balance of power in favor of the regime. It has also aided Assad’s forces in making considerable territorial advances against the Syrian National Coalition as well as Islamist groups, like IS and the al-Nusra Front, which is al-Qaida’s Syrian offshoot.

The Russian-Syrian alliance dates back to the Cold War when the Soviet Union provided Syria with economic and military assistance. The Suez War in 1956 and the Syrian Revolution in 1966 strengthened the partnership between the two entities. This partnership was enshrined by the Treaty of Friendship and Cooperation, which the two nations signed in 1980.

Russian President Vladimir Putin and Bashar al-Assad have grown closer since both men came to power in 2000. By mid-2000, Russia began to expand its military and strengthen its relationship with Syria through the sale of arms. Following the ousting of Libya’s Moammar Gadhafi during the 2011 Arab uprisings, Russia, which had a lucrative arms trade with Libya, began looking for new clients. Russian warplanes have aided Assad in killing hundreds of thousands of opposition fighters and civilians since the start of the conflict. Moscow also funds and equips mercenaries to help fight anti-Assad forces.

However, despite Assad’s chemical weapons attack in April 2017, that killed at least 80 people, Moscow shows unwavering economic and military support for its ally.

Syria’s civil war has expectedly divided the U.S. and Russia into opposing camps. The United States is allied with the Syrian Democratic Forces (SDF), a multi-ethnic and multi-religious umbrella militia group that is dominated by the Syrian branch of the Kurdistan Workers’ Party (YPG). Russia, Turkey and Iran support the Syrian regime in opposition of the SDF and the U.S.

Where, perhaps, the parties can find common ground is in the fight against IS. Russia has been instrumental in Assad’s ability to shore up power in the last two years and seems unlikely to withdraw support for the Assad regime anytime in the near future. The U.S. would be wise to remember the established partnership between Moscow and Damascus and to choose its battles carefully. Containing IS and removing Assad from power are likely incompatible goals in the near future.