The U.S. "didn't try" to stop the "catastrophic" Turkish invasion of northern Syria last month, according to a sharply critical internal memo sent by a top U.S. diplomat and obtained by the New York Times.

Why it matters: The diplomat, deputy U.S. envoy to the anti-ISIS coalition William Roebuck, said the U.S. had abandoned its Kurdish allies to a Turkish onslaught that involved "war crimes and ethnic cleansing." Those concerns have been widespread in the Pentagon and State Department but not stated publicly by senior officials. Roebuck sent the memo on Oct. 31 to the U.S. envoy for Syria policy, James Jeffrey, and to more than 40 other officials who work on Syria issues.

Key excerpts:

“Turkey’s military operation in northern Syria, spearheaded by armed Islamist groups on its payroll, represents ... what can only be described as war crimes and ethnic cleansing.”

“One day when the diplomatic history is written 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.”

"The decision to stay is a good one, even if the ‘protection of the oil’ rationale plays into toxic Middle Eastern conspiracy theories that will need to be lanced with careful, sustained messaging reinforcing the truism that Syria’s oil is Syria’s and for the benefit of the Syrian people."

What to watch: Turkish President Recep Tayyip Erdoğan will visit the White House on Nov 13.

Go deeper: Baghdadi raid depended on international ties Trump has spurned