This week, the United States and Turkey achieved a significant breakthrough. After many months of negotiations, the two countries agreed to coordinate on a joint buffer zone in northern Syria. This development offers a rare and fleeting opportunity for President Trump to step back from the brink of disaster. The president can salvage his Syria policy by making clear the United States will stick around to defend its vital national interests there.

Ever since Trump announced by tweet last December that he was withdrawing all U.S. troops from Syria, without consulting most of his military commanders, the United States’ Syria strategy — especially in the northeast — has been a muddle. To his credit, the president partially walked back the decision, announcing in February that a small, residual force would remain to keep the Islamic State down, keep our partnership with the Kurds and keep an eye on Iranian forces.

But the U.S.-Turkey rift, pushed to the breaking point by Turkish threats to unilaterally invade northeast Syria, risks turning that ambiguous U.S. policy into a total failure. If the Turks invade, the Kurds might ally with Syrian President Bashar al-Assad, to the advantage of Iran and the Islamic State. Any remaining U.S. leverage to push for an acceptable political solution would vanish.

After months of scant progress, the State Department announced Wednesday that Turkey and the United States agreed to coordinate establishment of a safe zone on the Syrian side of the border as a “peace corridor,” starting with a joint operations center inside Turkey to work out the details.

“The plan is for U.S. troops to work to patrol with Turkish forces to ensure security in the safe zone,” a senior State Department official told me, cautioning that the details aren’t final. “By strengthening U.S.-Turkish cooperation in the one area of the Syria conflict where we were at odds, this agreement also advances broader U.S. objectives for a resolution of the Syria conflict.”

If the United States wants to have any influence over what happens next in Syria, it will need to resolve its rift with Turkey, which has complex and serious causes. The Syrian Kurds, our allies whom we armed to fight the Islamic State, must be assured that Turkish forces won’t slaughter them. The Turks must be assured that the Kurds will withdraw from the border. We are not there yet. But this interim statement is a crucial step in the right direction.

What the administration won’t acknowledge, but what military and congressional leaders know, is that this entire scheme depends on keeping most of the current 900 or so U.S. troops still on the ground in Syria and maybe even sending in a few hundred more, Sen. Lindsey O. Graham (R-S.C.) told me.

Graham said Trump must publicly come out in support of the safe zone and promise the United States will play its role.

“The president has to let our allies know that we are not just going to abandon northeast Syria,” he said. “People are confused. It is now time to clear up the confusion. People don’t follow an uncertain trumpet.”

The idea of committing hundreds of U.S. troops to long-term safe-zone duty on the Syrian border — much less adding hundreds more — is a political hot potato. Cue the cries from the far left and far right, warning that Trump’s war hawks want to drag him into another Iraq-style invasion. In fact, no one is calling for that.

Graham told me military commanders on the ground favor bolstering the troop presence “by hundreds, not thousands,” to make the safe-zone idea work without sacrificing the counterterrorism mission or pushing the Kurds into the arms of the Assad regime.

Meanwhile, the Islamic State has “activated resurgent cells” in Syria, according to a Defense Department Inspector General report released this week. Kurdish forces need more U.S. training and equipment, not less, the report stated. There are 45,000 Islamic State supporters in an internally displaced persons camp called al-Hol who are largely unsupervised, and the Islamic State is recruiting inside the camp, the report warned.

Graham outlined the risk of withdrawing completely. Imagine that one year from now, the Islamic State is rampaging across Syria, the Kurds have joined with Assad, and Iran has expanded its presence in Syria. That would be a tough national security record for Trump to run on, Graham pointed out.

“From a political point of view, if ISIS comes roaring back because we withdraw, President Trump owns that, and it will undercut the argument that he is any different from President [Barack] Obama,” said Graham.

It’s true that Trump campaigned on getting the United States out of foreign wars. But he also campaigned on not repeating the mistake Obama made in Iraq when he completely withdrew and left a vacuum the Islamic State filled. If Trump must choose only one of those promises to keep, he should pick the one that keeps us safe.

This is not about military adventurism, regime change, oil or any other of the straw men routinely attacked by those who reflexively oppose any use of the military in foreign policy. This is about protecting our vital national interests, which include fighting extremism and trying to prevent Syria from becoming an even greater source of instability and human tragedy than it is now.

Read more: