One day he was inviting Mr. Erdogan to visit the White House; the next he was threatening to “totally destroy and obliterate” Turkey’s economy if it crossed a line that he never defined.

Mr. Erdogan just kept going.

Mr. Trump’s error, some aides concede in off-the-record conversations , was entering the Oct. 6 call underprepared, and then failing to spell out for Mr. Erdogan the potential consequences — from economic sanctions to a contraction of Turkey’s alliance with the United States and its standing in NATO. He has since threatened both, retroactively, and Secretary of State Mike Pompeo said later Monday that the president had signed an executive order authorizing sanctions on individuals or associates of Turkey’s government who “endanger civilians or lead to the further deterioration of peace, security and stability in northeast Syria.” But it is not clear whether Mr. Erdogan believes that poses a real risk.

The drama is nowhere near over. Out of necessity, the Kurds switched sides on Sunday, turning their backs on Washington and signing up with President Bashar al-Assad of Syria, a man the United States has called a war criminal for gassing his own people. At the Pentagon, officials struggled with the right response if Turkish forces — NATO allies — again opened fire on any of the 1,000 or so Americans now preparing to retreat from their positions inside Syria. Those troops are trapped for now, since Turkey has cut off the roads; removing them may require an airlift.

And over the weekend, State and Energy Department officials were quietly reviewing plans for evacuating roughly 50 tactical nuclear weapons that the United States had long stored, under American control, at Incirlik Air Base in Turkey, about 250 miles from the Syrian border, according to two American officials.