Instead, Iran’s fiery act of vengeance seemed to be a message aimed at both the Trump administration and Saudi Arabia. (The six ballistic missiles used by Tehran against ISIS, with a range of 700 kilometers, could reach major Saudi cities.) The kingdom has become emboldened regionally and escalated its anti-Iran rhetoric thanks, in part, to Trump’s message of seemingly unconditional support.

At the same time, Trump’s apparent willingness to use military force against Syrian President Bashar al-Assad and his chief supporters risks sparking a widening confrontation, while distracting from what he insists is his top priority: defeating ISIS in Iraq and Syria. This, from a president who campaigned, in part, on a pledge to avoid direct U.S. involvement in the Syrian conflict. Now, Trump has become a major player in an exploding regional proxy war that could determine the Middle East’s post-war dynamics.

Sunday’s events place the danger of escalation and the staggering complexities of the phalanx of alliances in Syria into stark relief. The confrontation began when U.S.-allied fighters with the Syrian Democratic Forces (SDF), a coalition of Kurdish, Sunni Arab, Christian, and Turkmen rebel groups anchored by the largely Kurdish People’s Protection Units (YPG), came under attack from “pro-Syrian regime forces” in a town south of Tabqa, the site of a strategic dam that had been under ISIS control for several years until the SDF captured it in May. (Over the past year, the SDF and YPG had largely avoided confrontation with Syrian forces—a modus operandi that may be changing as Assad and his allies grow bolder in the race for control of southeastern Syria.) The Pentagon coordinates its activities in Syria with Russian forces, and U.S. officials said they contacted their counterparts on a “de-confliction” phone line asking them to intervene with Syrian forces to stop the attacks. But two hours later, the Pentagon said, a Syrian-regime jet dropped bombs near SDF fighters, and it was shot down by a U.S. Navy plane.

Afterwards, the Pentagon said it would protect the Syrian rebels it has been training and arming for more than year to launch the assault on ISIS in Raqqa. “The coalition’s mission is to defeat ISIS in Iraq and Syria,” the U.S. statement said. “The coalition does not seek to fight [the] Syrian regime, Russian, or pro-regime forces partnered with them, but will not hesitate to defend coalition or partner forces from any threat.”

And foremost among those threats, in the eyes of the Trump administration, is Iran. While Trump has changed his mind on a number of foreign-policy questions since taking office, he has been consistent in his belief that Iran, the world’s main state sponsor of terrorism, poses the greatest threat to U.S. interests in the Middle East. He’s surrounded himself with advisers like Defense Secretary James Mattis and National Security Adviser H.R. McMaster, battle-hardened former military commanders who want to take an aggressive approach to contain Iran.