SYRIA HAS seen a dangerous escalation of hostilities involving outside powers, including the United States. Last week a force loyal to the Syrian government of Bashar al-Assad attacked U.S.-backed Kurdish forces, drawing a devastating American air and artillery response. Three days later, Israel, Syria and Iran exchanged blows after an Iranian drone crossed into Israel; an Israeli plane was shot down. Meanwhile, Turkey is continuing to assault a Kurdish enclave on its border and threatening to attack another one where U.S. troops are present; and Syrian government and Russian forces are pounding rebel positions east of Damascus and in the northern province of Idlib, including with chemical weapons.

Far from winding down, Syria’s civil war is threatening to trigger direct conflicts between the United States and Turkey, Israel and Iran, and even the United States and Russia. These threats can be defused only by high-level diplomacy backed by the credible threat of force. So far, the Trump administration’s response looks underpowered.

What all the fighting has in common is the involvement of Russia — as a participant, silent partner or cease-fire broker. Russian forces are backing the Assad regime’s offensives, and they, along with Iran, may have supported the attack on America’s Kurdish allies east of the Euphrates River. Russian ruler Vladi­mir Putin gave Turkey a green light to launch its offensive against the Kurds, and his phone call to Israeli Prime Minister Benjamin Netanyahu on Saturday put a stop to hostilities between Israel, Syria and Iran.

Mr. Putin is seeking to establish Russia as the dominant power in Syria and, by extension, a major player in the Middle East — all at the expense of the United States. His attempt to stage a conference supplanting the U.N.-sponsored peace process for Syria largely flopped last month. But he has established Russia as the arbiter of Syria’s multiple conflicts, capable of fueling them or shutting them down.

As elsewhere, one of Mr. Putin’s tools is bold duplicity. He cut deals for “de-escalation zones” in northern and southern Syria with Turkey and the United States, then allowed Syrian and Iranian forces to shred them. Russian officers stayed in contact with their American counterparts during the attack on the Kurds last week, as if they were not involved; but outside analysts believe that Russia helped to plan the attack and that Russian “contractors” participated in it.

Though U.S. forces appear able to tactically defend the pieces of eastern and southern Syria their allies control, the Trump administration lacks the leverage to pursue its stated goals in the country, which are to prevent a resurgence of the Islamic State, stop Iran from entrenching its forces there and promote a new political order that excludes the Assad regime. Israel, too, is failing to achieve its strategic objective, which is to drive out Iranian forces. On the contrary, Russia, Iran and Turkey are betting they can eventually push the United States out of Syria, allowing Turkey to overrun the Kurds, the Assad regime to regain full political control and Iran to dig in on Israel’s northern border. Unless the Trump administration is prepared to make a larger military and diplomatic commitment, that is the most likely outcome.

Read more here: