BEIRUT – After the United States heavily retaliated against pro-Syrian government forces advancing on an American-controlled base, Secretary of Defense Jim Mattis repeated the standard justification for U.S. action in the country: “It was self-defense. Obviously, we are not getting engaged in the Syrian civil war.”

The U.S. is already engaged in the war (self-defense is a claim made by those with skin in the game), but what this likely meant is that, according to the Pentagon, Washington is not “looking for a conflict with the regime.” In short, the U.S. show of force in Deir Ezzor province last week did not change Washington’s engagement strategy in Syria: It will defend its interests, no matter who threatens them.

Questions remain, however, over what (or who) falls under the umbrella of U.S. interest in Syria now that the so-called Islamic State has been largely pushed out of its strongholds, and over whether or not Russia and pro-regime forces will adhere to the Deir Ezzor “de-confliction line” agreed upon by Moscow and Washington.

Though the battle against ISIS has largely finished, last week’s attacks show that Washington’s military presence in Syria “will not go untested,” Jonas Parello-Plesner, senior policy fellow at the Hudson Institute, told Syria Deeply.

Wednesday’s attack in the town of Khusham, southeast of the provincial capital of Deir Ezzor, was 5 miles (8km) east of the de-confliction line. The U.S. and Russia established the line last year to avoid further confrontation between their forces and allies, and to delineate between their areas of operation. The line begins in the town of Tabqa and runs along the Euphrates River across Deir Ezzor toward the border with Iraq.

The U.S.-led anti-ISIS coalition has exerted what it calls “its non-negotiable right to act in self-defense” several times before.

On June 8 last year, the U.S. Air Force shot down a “pro-Syria regime” drone that dropped one of its weapons near coalition partner forces near Tanf. On June 18, the U.S. shot down a Syrian regime SU-22 fighter jet that dropped bombs near U.S-backed Syrian Democratic Forces (SDF) south of Tabqa. On June 20, coalition forces shot down another drone, “after it displayed hostile intent and advanced on coalition forces.” In September, Russian airstrikes hit SDF positions nearby to where the recent attack took place, killing at least one U.S.-backed fighter. In December, U.S. jets intercepted Russian jets that violated the “de-confliction line” east of the Euphrates river.

These instances have largely been in defense of coalition allies, mainly the SDF. While Washington has previously flip-flopped about its support to the SDF after the battle against ISIS, the Trump administration is now showing “determination to … hold onto and defend the SDF zone in northern and eastern Syria,” said Nicholas A. Heras, the Middle East security fellow at the Center for a New American Security.

“Pure and simple, anywhere the SDF controls in eastern Syria is considered by the Americans to be no-go zones for [President Bashar] Assad and his allies,” Heras said. “Every inch of the soil under the SDF is now a de facto American military base, and will be defended as such.”

Washington’s main reasons for staying in Syria are to “secure the complete defeat of ISIS,” carry out stabilization efforts and maintain “leverage on getting the political talks in Geneva back towards any meaningful progress,” Parello-Plesner said.

However, last week’s attack consisted of a much greater show of force from both the U.S. and pro-regime forces than in previous instances, suggesting that U.S. interests in Syria may increasingly clash with those of pro-regime forces and Russia. The coalition said the attack was likely an attempt to capture the lucrative gas and oil fields in the SDF-controlled area.

“Damascus is furious that the Americans are backing the SDF coalition there, essentially boxing Assad out of his oil. There is every incentive for Assad and his partner forces, be they Iranian proxies or Russian mercenaries, to seek to conquer the energy resource-rich SDF zone,” Heras said.

On the eastern banks of the Euphrates, U.S.-backed forces control a number of oil fields and gas plants that the SDF seized from ISIS last year. Among them is the Conoco natural gas facility, which had the largest capacity of any gas field in Syria before the conflict.

“Pro-regime forces want to recapture Syria’s critical infrastructure and natural resources in order to rehabilitate the Assad regime, provide economic concessions to repay Russia and Iran for their military support, and as a stepping stone toward their ultimate goal of reconquering the country,” said Jennifer Cafarella, senior intelligence planner at the Institute for the Study of War. “They also want to expel U.S. forces, a goal the Assad regime is messaging frequently.”

The scaled-up U.S. response was “adjusted to the nature and scale of the attack,” Cafarella said, adding that it also shows that “U.S. force protection strikes in Syria thus far have clearly not deterred pro-regime forces.”

U.S. officials estimated that as many as 500 pro-government fighters took part in the attack, “supported by rocket and mortar systems and Soviet-era tanks,” the Washington Post reported, citing an unnamed U.S. military official.

Moscow initially denied involvement in last week’s attack, but several reports claimed that forces from the Russian private military contractor Wagner participated. The Conflict Intelligence Team, which monitors social networks for information about Russians in Syria, said it confirmed the deaths of at least six Russians as of Tuesday.

“Russia had both incentive and means to support the attack. The Russians may well be testing whether they will get away with sending in irregular forces like Wagner contractors,” Cafarella said.

Last year, Syria agreed to give a Russian oil company 25 percent of oil and gas profits from areas that private Russian security contractors helped Damascus retake.

However, according to Nawar Oliver, a military researcher, mapper and analyst at the Omran Center, oil is only “a small part of the story.”

In addition to its rich resources, eastern Deir Ezzor’s proximity to Syria’s border with Iraq, and its demographics make it even more significant for Syria and its allies, particularly Iran, Oliver said. The area is also deeply tribal, and “these tribes can mobilize in the future by whoever controls them”; controlling Deir Ezzor would allow the government and Iran to secure almost the entire frontier with Iraq.

In this context, last week’s attack falls well within the scope of Secretary of State Rex Tillerson’s recently outlined policy for Washington’s stay in Syria, which includes ensuring “Iranian influence in Syria is diminished [and that] their dreams of a northern arch are denied.”

Washington’s goals in Syria are not limited to fighting ISIS, and while the recent show of force in Deir Ezzor may not have signaled a change in engagement, it raises doubts about how the U.S. can protect its interests without further clashes with pro-government forces.

Hashem Osseiran contributed reporting to this article.