But this much already is clear: Mr. Trump ignored months of warnings from his advisers about what calamities likely would ensue if he followed his instincts to pull back from Syria and abandon America’s longtime allies, the Kurds. He had no Plan B, other than to leave. The only surprise is how swiftly it all collapsed around the president and his depleted, inexperienced foreign policy team.

Day after day, they have been caught off-guard, offering up differing explanations of what Mr. Trump said to Mr. Erdogan, how the United States and its allies might respond, and even whether Turkey remains an American ally. For a while Mr. Trump said he acted because the Islamic State was already defeated, and because he was committed to terminating “endless wars” by pulling American troops out of the Middle East. By the end of the week he added 2,000 — to Saudi Arabia…

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.