Mr. Roebuck’s memo appears to be the first formal expression of dissent on Syria from a Trump administration official to be made public. Pentagon officials voiced alarm by the sudden shift in Syria policy, but top officials never made their views public.

Mr. Roebuck’s memo also comes as the president already has expressed disdain for some State Department officials because of their testimony in Congress during the impeachment inquiry over Ukraine policy.

For nearly two years, Mr. Roebuck has worked on the ground in northern Syria with Syrian Kurdish and Arab military and civilian officials who make up what is called the Syrian Democratic Forces. Mr. Roebuck has been an important interlocutor with Mazlum Kobani, the Syrian Kurdish military commander whose fighters have worked closely with American Special Operations forces to combat the Islamic State.

Mr. Roebuck focused his harshest criticism on Turkey’s military offensive and specifically on Turkey’s deployment of Syrian Arab fighters in its vanguard force. Mr. Roebuck added his voice to accusations by human rights groups that these fighters have killed Kurdish prisoners, including one of them lying on the ground with his hands bound behind his back, and committed other atrocities as they emptied major Kurdish population centers in northern Syria.

“Turkey’s military operation in northern Syria, spearheaded by armed Islamist groups on its payroll, represents an intentioned-laced effort at ethnic cleansing,” Mr. Roebuck wrote, calling the abuses “what can only be described as war crimes and ethnic cleansing.”

“One day when the diplomatic history is written,” he said, “people will wonder what happened here and why officials didn’t do more to stop it or at least speak out more forcefully to blame Turkey for its behavior: an unprovoked military operation that has killed some 200 civilians, left well over 100,000 people (and counting) newly displaced and homeless because of its military operation.”

Mr. Roebuck continued, “To protect our interests, we need to speak out more forcefully, publicly and privately, to reduce the blame placed on the U.S. and to highlight the Turkish responsibilities for civilian well-being.”