On Thursday, the United States deepened its involvement in the Syrian Civil War in ways that may only gradually become apparent. In targeting a convoy that Secretary of Defense James Mattis said included Iran-backed militiamen as well as Syrian regime forces, the U.S. apparently, for the first time since the conflict began six years ago, attacked foreign fighters allied to the Syrian government. The same incident also represented the second time the U.S. military has deliberately targeted Assad’s own forces, which the Trump administration struck last month in retaliation for the Syrian government using chemical weapons against civilians. Under Barack Obama, U.S. military operations in Syria were directed at ISIS and al-Qaeda affiliates. Now two additional factions in the multifaceted civil war are in America’s crosshairs.

The development is in part a reflection of the fact that Donald Trump, who as a presidential candidate promised to focus solely on ISIS in Syria, has as president taken a surprisingly hard line on the Assad government. But it’s also reflective of a broader dynamic: As ISIS loses strength and territory in Syria, the endgame of the civil war is drawing nearer and the various powers engaged in that struggle are shedding a common enemy. The result is a race to carve out spheres of influence—and the United States under Trump appears to be getting in on the action.

Consider the context of Thursday’s strikes. U.S. warplanes hit a pro-Assad military convoy that ignored warnings to steer clear of a base where American and British special-operations forces are training Syrian rebels to fight ISIS. In 2016, Russian aircraft twice bombed that same base. But the Obama administration, out of concern about getting drawn deeper into the Syrian conflict, did little to deter the Russians.

By striking pro-Assad forces that were encroaching on a hub for the U.S. campaign against ISIS, the Trump administration was “establishing a deterrent in Syria that we frankly didn’t have,” said Faysal Itani, a scholar of the Syrian conflict at the Atlantic Council. “We lost it over [Barack Obama’s decision in 2013 not to enforce his red line on] chemical weapons, we lost it over the fact that when we have guys on the ground who share our military agenda, we don’t protect them, and we give the impression that you can violate with impunity U.S. counterterrorism strategy.”

The strikes asserted “more conventional power politics that were not really a part of the Obama administration’s thinking” in Syria, Itani told me. They sent a message that the area around the base—al-Tanf in southeastern Syria, near the borders with Iraq and Jordan—was an “American sphere of influence and area of operations.”

The problem is that several factions in the conflict—most prominently Iran—aren’t willing to grant the Americans that plot of land. If the U.S. military did indeed strike Shiite militiamen affiliated with Iran this week, the bombing is a “very big deal,” Charles Lister, an expert on the Syrian Civil War at the Middle East Institute, told me—and not only because it could complicate cooperation between such militias and U.S. and Iraqi forces in the fight against ISIS across the border.

For days now, Lister said, Iran-backed militias have been “seeking to intimidate U.S.-backed anti-ISIS forces in southern Syria” and Iranian state media has been running reports warning of a “‘US plot’ to establish a buffer zone in southern Syria ‘to protect terrorists.’”

“Iran doesn’t want a single American in Syria and they’ve got an extensive track record of fighting Americans next door in Iraq,” Lister wrote by email. “Just as Assad and Iran look to be winning the ground war in Syria, the U.S. and its Syrian opposition partners in the south have intensified their own anti-ISIS activities, exerting more and more of an influence and presence in the south.”

“Simultaneously,” Lister added, “the U.S. is making no secret of its intent in northeastern Syria, where [the U.S. military] now operate[s] at least two airbases. All of this represents a very serious challenge to Iranian hard-won gains in Syria and the prospect for a contiguous land corridor from Tehran, to Baghdad, to Damascus, and to Beirut. That’s Iran’s ultimate dream objective in the Middle East and U.S. zones of influence in Syria’s north and south threaten that very much.”

Reuters/ Institute for the Study of War

The al-Tanf base is located in a sparsely populated desert region known as the Badia; in the map above, the base is the pink splotch in the sea of white. But it also happens to be near oil reserves, the Damascus-Baghdad highway (which Iran has used as a supply route for weapons), and the Syrian city of Deir Ezzor, where the Islamic State might make its last stand if the group is uprooted from its stronghold of Raqqa. The retreat of ISIS and peace talks in Kazakhstan, which recently produced a Russian/Iranian/Turkish plan to divide the Syrian battlefield into into “de-escalation zones,” have made the quest to control the area more urgent. Hence the mobilizing of forces and rattling of sabers there this week.

As The New York Times reports:

The crowded field of foreign participants in the Syrian conflict are all maneuvering for influence in and around de-escalation zones that Russia and Turkey have sought to establish. One of them is southern Syria, where the United States, Jordan and Israel are eager to prevent Iranian-backed groups, especially Hezbollah, from expanding a foothold. ...

The area is strategically important to the United States, which wants to stabilize Iraq where it has a long-term military and political investment, and to Russia, which wants to strengthen the Syrian government’s control of as much territory as possible.

And the area is critical to Iran, which wants secure corridors from its borders to the Mediterranean to reinforce its influence in Lebanon and maintain an ability to challenge Israel. All claim to be battling the Islamic State but have refused to collaborate.

Ahmad Majidyar, an Iran expert at the Middle East Institute, argues that the geopolitical power struggle on display in Syria this week is also playing out in Iraq, where “Iran-backed militia forces have launched a new military offensive ... to seize more territory from the Islamic State in western Mosul and control the land corridor from the strategic city of Tal Afar to the Syrian border.”

“It is too soon to say whether [Thursday]’s strikes mark a significant shift in ... U.S. policy in Syria—although it is the first time the U.S. military is providing air support to rebel groups attacked by pro-Assad forces—or the strikes were a one-time action for force protection purposes,” he wrote this week. “But one thing is clear: As the common enemy—the Islamic State—is losing ground in Iraq and Syria, the prospect of dangerous and direct confrontation between the Iran-backed militia forces and the U.S.-led coalition forces in Syria and Iraq is on the rise.”

But such risks of confrontation are the unavoidable price of committing to defeat the Islamic State in Syria, Itani said. Say the United States succeeds in removing ISIS from Raqqa and Deir Ezzor. How will it prevent the terrorist group from bouncing back? The only option is for the U.S. or its allies to hold ISIS’s former territory. And that requires staking out spheres of influence in Syria.

“I never thought it made sense to compartmentalize the ISIS problem from the geographies and the geopolitics and the military realities of the war,” Itani told me. “Now I would have understood a position that said, ‘ISIS is just not worth it. We’ll bomb them a bit, we’ll weaken them, we’ll cooperate with our allies, but we’re not going to degrade and destroy them and set this bar” so high. Donald Trump, however, hasn’t taken that position. He’s set the bar high.

That decision “doesn’t mean you have to go invade [Syria] with a quarter million troops,” Itani said. “But it does mean, yeah, you have to sit down and spend a lot of energy and time on the problem. It’s very consuming.” Meanwhile, just days after beating back the Syrian military and its Iran-backed allies, the American president has arrived at the first destination of his first official foreign trip: Saudi Arabia, otherwise known as Iran’s archenemy.