I returned this year from military service in northeastern Syria, where the U.S. has supported local Kurdish, Arab and Syriac Christian militias in a grim campaign to dislodge Islamic State. Now refugees are returning to their homes, and locals are starting to rebuild after five years of fighting and nightmarish ISIS rule. In most places I was greeted by civilians thankful for the U.S. presence. I’ll never forget the little girl who ran up in a recently liberated market town and hugged my leg, refusing to let go.

But this fragile rose blooming in the desert will likely be crushed if the U.S. departs.

With peace finally in sight, Turkey’s President Recep Tayyip Erdogan last week threatened to invade northern Syria and “cleanse” the region of our Kurdish partners. After a phone call with Mr. Erdogan, President Trump tweeted that the U.S. may soon pull all 2,000 troops from Syria. The best way for the U.S. to avoid dishonor and calamity is to walk back this policy shift and publicly commit to safeguarding its Kurdish partners until a durable peace agreement can be reached.

The U.S. and the West have quietly relied on the Syrian Kurds to sacrifice their young men and women by the thousands to defeat the Islamic State. Thanks largely to their efforts, ISIS in Syria has gone from a fearsome juggernaut to a ragged band of die-hards trapped in a shrinking patch of wasteland.

The partnership dates to 2014, when the Kurds mounted an inspiring last-ditch defense of Kobani against ISIS’ advance. The Kurds could have halted and focused on consolidating their own territory, but at America’s urging they expanded their effort against ISIS. In a coalition with Arabs and Syriacs of the Euphrates River Valley, they’ve swept south to dislodge ISIS from one-third of Syria.

I met many young Syrian fighters of all ethnicities, who under Kurdish leadership were determined to liberate their lands from Islamist despotism. In Raqqa and along the Euphrates I witnessed firsthand as steady trains of field ambulances carried Kurdish casualties in battle after battle. The Kurdish-led civil administration does the heavy lifting of guarding hundreds of ISIS’ most dangerous foreign fighters while their home countries drag their feet on extradition.

The West owes them a debt for the price they’ve paid. Instead, a U.S. departure would threaten them with disaster. Already Mr. Erdogan has directed two invasions of Syrian border regions—in 2016 north of Aleppo and this January in the northwestern Afrin region. Mr. Erdogan labels America’s Syrian Kurdish partners “terrorists,” links them with separatist rebels in Turkey, and suggests resettling their land with Arab refugees from elsewhere in Syria.

The Kurds have earned a reputation for fighting bravely, but without U.S. air power their prospects against a modern army with a robust air force would be grim. An invasion would force Kurdish forces to pull back from the front lines against the remnant of ISIS, allowing the jihadists to regroup and proliferate. It would likely spawn a fresh humanitarian catastrophe, including areas that have been mostly spared the worst of Syria’s civil war.

I heard frequently from Kurds about their fears of ethnic cleansing should Turkey invade. Invasion would also leave the door wide open for the Assad regime to launch an assault with help from Russia, Iran and Hezbollah. I witnessed just such an incursion attempt by regime elements this February, while the Kurds were distracted with Ankara’s invasion of Afrin—an incursion repelled only by U.S. firepower.

A pullout would harm U.S. interests as well. It would shred America’s credibility as a counterterrorism partner world-wide, while abandoning a strategic area and making it harder to check jihadist, Iranian and Russian ambitions. Mr. Trump should make clear the U.S. stands with the Syrian Kurds and won’t permit a Turkish invasion. No one wants American troops to stay in Syria forever, but U.S. interests and honor demand that they stay for now.

Mr. Meyerson, a former U.S. Marine, is a student at the University of Pennsylvania’s Wharton School of Business.