Russian officials have responded with outrage to Thursday’s U.S. missile strikes on a Syrian military air base, vowing to increase the Kremlin’s military backing of Syria’s President Bashar Assad, a sign of growing tensions between America and Russia.

Russia called the barrage of Tomahawk missiles targeting Shayrat airfield a violation of international law, and on Friday suspended a key U.S.-Russia agreement to coordinate their air operations over Syria ― something intended to prevent accidental conflict. Over the weekend, Russia deployed a warship to its naval base on the Syrian coast, what some observers view as aggressive posturing from the Kremlin.

The U.S. strikes, conducted in response to a toxic gas attack that killed scores earlier this week, have posed one of the most significant tests of Russia’s relationship with Syria since the conflict began. They have also highlighted the lengths that Russian President Vladimir Putin is willing to go to support Assad no matter what actions the latter is implicated in.

The roots of Russia’s relations with Syria go back decades. They grew after Bashar Assad’s father Hafez Assad came to power in 1970. The elder Assad, an air force officer who spent time in the former Soviet Union learning to fly MiG fighter jets, sought greater military cooperation with the Kremlin. Hafez also set up a one-party state that mirrored elements of the Soviet Union, including a powerful network of intelligence agencies and secret police.

Throughout the Cold War, the Soviet Union supplied Syria with arms and developed closer bilateral relations. In 1971, the two nations also signed an agreement giving the Soviet Union control of a naval base on Syria’s Mediterranean coast in the city of Tartus. The base is currently the last Russian-controlled port on the Mediterranean, giving it both symbolic and strategic importance to the Kremlin.

In January, the two countries struck a deal to expand the Tartus base, in an agreement that will allow Russia to host as many as 11 warships.

It’s through Tartus that Russia has been shipping Assad a wide range of arms, which have kept him in power during the more than six years of civil war that has killed hundreds of thousands of people. In recent years, Russia has also deployed air defense missile systems to Tartus, and following the U.S. strike this week, has also vowed to help Syria expand its air defenses.

Russia’s military support for the Syrian government, most notably its airstrike campaign that began in September 2015, has caused a near total reversal of opposition gains on the ground, allowing, human rights groups say, Assad to act with increased impunity under the safety of Russian backing.

Putin has used Russia’s presence in Syria as a means of shoring up domestic support to show that the nation is still relevant in geopolitics. For years, Russia has limited the United States’ ability to exert influence in Syria and pressure Assad from power.

At the United Nations Security Council, Russia has vetoed even mild resolutions aimed at Assad, while their military presence and the risk of greater conflict deterred former President Barack Obama from intervention.

But Putin’s support for Assad has also come at significant cost to Russia. The Kremlin has spent hundreds of millions in Syria, been forced to downplay its military casualties and become increasingly tied to a government that is a pariah to most of the world. The chemical attack this week also severely damaged Russia’s legitimacy after it brokered and acted as de facto insurer of a 2013 agreement to remove Assad’s chemical weapons stockpiles.

As Putin stands firm on backing Assad’s regime, there is now an increased risk and uncertainty over what may happen if future U.S. actions in Syria come into open conflict with Russian interests. The days since the U.S. strike have shown that the Kremlin does not want to be seen as backing down, but in doing so Putin is now further entangled in a war with no end in sight that has just become even more complex.

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