A Kurdish fighter views smoke from a coalition airstrike. (Goran Tomasevic/Reuters)

THE UNITED STATES is committed to defeating the Islamic State in Iraq and Syria, but as that goal nears realization, another strategic question looms: What security order will replace it, and which of the outside powers enmeshed in the region will stand behind that order? The Trump administration doesn’t appear to have a strategy for that, but others clearly do — which helps to explain the incidents over the weekend in which the United States downed a Syrian government warplane , while Iran fired intermediate-range missiles from its territory at Islamic State targets in eastern Syria.

Though the two incidents were nominally unrelated, they have a common cause: the drive by Iran and Russia, along with their Syrian and Iraqi Shiite clients, to dominate the space that will be left when the Islamic State is driven from its capital of Raqqa in eastern Syria, which is under assault from U.S.-backed Kurdish and Syrian Arab forces. At stake are both Syria’s oil-producing area to the south of Raqqa and a land corridor between Baghdad and Damascus that Iran aspires to control. Russia, for its part, hopes to drive the United States out of the region.

In the past month, U.S.-backed forces in Syria’s southeastern corner have come under pressure from Iranian-backed Shiite militias. U.S. commanders have twice bombed convoys that entered an exclusion zone around a border town where American advisers are based and they have destroyed a drone . The Syrian fighter bomber shot down Sunday violated another exclusion zone around the forces surrounding Raqqa. Meanwhile, Iran’s missile attack, which it said was in response to the Islamic State’s recent raid on the parliament building in Tehran, was a bold assertion of its willingness to escalate militarily in Syria and maybe elsewhere in the region.

Syria and Iran may calculate that the Trump administration can be induced to abandon the area rather than risk being dragged into a war in the Syrian desert unrelated to the Islamic State. Russia’s loud protests about the downing of the fighter — and its threats to challenge U.S. planes over Syria — show that Moscow is more than ready to support this gambit.

The United States doesn’t have a strategic reason to control southern and eastern Syria, but it does have a vital interest in preventing Iran from establishing a dominion from Tehran to the Mediterranean with Russia’s support. That would pose an existential threat to Israel, which is already struggling to prevent Iranian infiltration of Syrian territory adjacent to the Golan Heights, and would undermine U.S. allies in Jordan and Iraq.

Countering Iran and Russia requires tactical defense by U.S.-backed forces, like that recently ordered by commanders on the ground. But it will also require a broader strategy to create a security order in the region acceptable to the United States and its allies. To achieve that, the administration may need to raise the military or economic pressures on Iran, Russia and the Syrian government while pressing for negotiations on a new Syrian political order. Not only should the United States reject Moscow’s bluffing about Syrian airspace, but also the Trump administration should make clear to Vladi­mir Putin’s regime that if it continues to ally itself with Iran in the region, it will forfeit any chance of resetting relations with Washington.