Despite President Donald Trump’s reluctance to get deeply involved in the Syrian civil war, the United States now finds itself in the middle of an escalating battle in the country’s south that last week led to a clash between the U.S. military and Iranian-backed pro-government forces. If he can seize the opportunity, Trump could deal a blow to Iranian regional influence and help save Syria in the process.

To hear the Trump administration tell it, the coalition airstrike May 18 near the al-Tanf base on Syria’s border with Jordan and Iraq was a one-off event. A statement from U.S. Central Command said that “pro-regime forces” had crossed into an “established de-confliction zone,” posing a threat to opposition forces and the U.S. troops who are training them.

But the skirmish near al-Tanf was not an isolated incident. According to officials, experts and rebel leaders on the ground, an ongoing and rapidly accelerating confrontation in that area was triggered by an offensive by Iranian-backed militias. Iran is trying to establish strategic control over territory creating a corridor from Lebanon and Syria through Baghdad to Tehran.

If successful, the Iranian campaign would drastically reshape the regional security situation, harm the fight against the Islamic State in the nearby city of Deir al-Zour and directly undermine U.S. efforts to train and equip an indigenous Sunni Arab fighting force, which is essential to establishing long-term stability.

In short, it’s a fight that the United States cannot and should not avoid. It’s also an opportunity for Trump to accomplish what his administration says it wants to do in the Middle East: Push back against Iranian aggression and expansionism.

So far, the White House doesn’t see it in that light. An official told me that the decision to strike regime and Iranian-backed forces last week was made by military commanders on the ground, not by the White House. The commanders have the authority to strike whenever they believe U.S. troops are under threat, the official said, stating that there has been no change in U.S. policy in Syria.

“There was no large, big-picture change that resulted in this scenario,” the official said.

The strikes did change Tehran’s calculus. The Middle East Institute’s Charles Lister said that the bombs hit a militia backed by the Iranian Revolutionary Guard Forces called Kataib Imam Ali. After the strikes, the Iranian FARS news agency reported that Iran will send 3,000 Hezbollah fighters to the al-Tanf region to thwart a “U.S. plot.”

A Syrian opposition rebel leader who works with the U.S. military said that while there are a mix of regime, Iranian and militia forces fighting in the area, the Iranians are in command of the campaign. Their first goal is to establish control over a security triangle that would give them free movement between the eastern Syrian towns of Palmyra and Deir al-Zour and Baghdad.

The Iranians’ second goal is to block the U.S.-supported rebels in al-Tanf from Deir al-Zour. If the rebels take the city from the Islamic State, it would be a huge boon for the Sunni opposition to the regime of Syrian President Bashar Assad.

Those who support the Syrian opposition in Washington also are noticing a shift in the U.S. approach toward confronting Iran in Syria. Whether that represents mission creep or a deliberate change in approach on a policy level is unclear — and ultimately irrelevant. Perhaps by accident, Trump is moving toward a Syria policy that is tougher on Iran and the Assad regime, and it’s having real effects on the ground.

“The United States has two major adversaries in Syria, that is Iran and ISIS. Both represent huge risks to U.S. national security and interests in the region,” said Mouaz Moustafa, executive director of the Syrian Emergency Task Force.

The battle for Syria’s south is on, and the Trump team must decide if the United States will play a decisive role. Trump could fulfill his promises to thwart Iran and bring greater stability to Syria — if he acts fast.

Josh Rogin is a columnist for the Global Opinions section of The Washington Post. He writes about foreign policy and national security.