Mr. Trump spoke with Mr. Erdogan on Monday, urging him to “stop the invasion” and agree to enact a cease-fire, Vice President Mike Pence said. Mr. Pence also announced that Mr. Trump had asked him to lead a delegation to Turkey.

But the actions were, in the minds of many American diplomats, a week late: The time to threaten sanctions was when Mr. Trump spoke with Mr. Erdogan by phone on Oct. 6, just before the invasion, in an effort to dissuade him from proceeding with the attack on Kurdish forces allied with the United States. Now, the invasion has resulted not only in the deaths of Kurdish allies, but also signs that it is benefiting Russia, Iran and the Islamic State.

Exactly what Turkey would need to do to end the sanctions was also unclear. Mr. Trump said nothing about bringing its troops back home. In a statement on Monday night, Secretary of State Mike Pompeo was somewhat more specific, warning Turkish officials that “to avoid suffering further sanctions imposed under this new executive order Turkey must immediately cease its unilateral offensive in northeast Syria and return to a dialogue with the United States on security in northeast Syria.” That sounded more like a cease-fire than a retreat from Kurdish-controlled territory.

Three top Senate Democrats — Senators Chuck Schumer of New York, Robert Menendez of New Jersey and Jack Reed of Rhode Island — warned in a joint statement that “strong sanctions, while good and justified,” would not be enough to reverse the damage. They called on Republicans to join them in a resolution that would force Mr. Trump to reverse his decision to withdraw American forces from Syria.

“President Trump should use this moment to step up, do the right thing and correct course,” they said. “He can listen to his national security and military advisers, and to the chorus of bipartisan voices in Congress and across the world who made it clear that this was a grave mistake worth reversing.”