A U.S-backed Syrian Manbij Military Council soldier passes a U.S. position near the tense front line with Turkish-backed fighters, in Manbij, Syria, on April 4, 2018. (Hussein Malla/AP)

John R. Allen’s Jan. 4 Friday Opinion essay, “Our gains against ISIS are now at risk,” clearly laid out why President Trump shouldn’t abandon our fight with the Islamic State by leaving Syria. Prematurely declaring victory would allow the terrorist organization to rebuild and again threaten our national security.

Despite the president’s claims, the Islamic State hasn’t been defeated. It remains a serious threat in Iraq and Syria, as do its affiliates in Yemen, Afghanistan and pockets of Africa, and it still attracts a significant following on the Internet. If the president withdraws U.S. forces from Syria now, he would be granting the terrorist organization a reprieve.

U.S. and Kurdish forces have cornered the Islamic State along the Iraq-Syria border. Without an American presence, the Kurds would be vulnerable not only to the Islamic State but also to a potential attack by Turkey. Rather than eliminating the remaining elements of the Islamic State, Kurdish forces would be distracted preparing to defend themselves against a Turkish onslaught.

I met Mr. Allen during a trip with other congressional leaders to Kabul in 2012. He has proved to be a thoughtful leader with considerable expertise in the region. The president would be wise to heed his advice and not abandon our fight against the Islamic State at this crucial moment.

Dianne Feinstein, Washington

The writer, a Democrat, represents California in the Senate.