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.