A memo from a U.S. diplomat on the ground in northern Syria sharply criticizes the administration for not doing more to stop Turkey’s invasion last month, saying the Turks committed war crimes and ethnic cleansing when given a free hand.

The memo was written by veteran diplomat William V. Roebuck and presents a blunt assessment of the impact of President Trump’s decision to pull U.S. troops away from the Syria-Turkey border, abandoning support for the Kurdish fighters who helped oust Islamic State group terrorists from the region.

Roebuck said the administration should have applied more pressure on Ankara, including sanctions and increased military patrols, which have worked to dissuade the Turks from invading in the past.

“It is a catastrophic sideshow [to the overall Syrian conflict] and it is to a significant degree of our making,” Roebuck said.

The New York Times obtained a copy of the Oct. 31 memo, which was sent to Roebuck’s boss at the State Department, James Jeffrey, the special envoy on Syria policy, and a number of other State Department, Pentagon and White House officials two weeks after Vice President Mike Pence met with Turkish President Recep Tayyip Erdogan and worked out a ceasefire that resulted in Russia filling the vacuum left by the U.S. departure.

Trump has argued Turkey was going to invade no matter what, and he pulled out U.S. troops to keep them out of harm’s way.

In an interview with Defense One earlier this week, Deputy Assistant Defense Secretary for the Middle East Mick Mulroy, who left the administration Friday, said he thinks keeping 50 U.S. troops in the region could have dissuaded Erdogan.

“I think it would have worked, to be honest,” Mulroy said. “It wasn’t perfect. But I think it was in everybody’s interest to actually have done that. Turkey’s security concerns would’ve been addressed. We would have prevented the need for an incursion.”

 “No one can deny that the situation in Syria is very complicated, and there are no easy solutions and no easy choices,” State Department spokeswoman Morgan Ortagus said. “There will always be a variety of opinions on how this complex situation should be managed. This administration’s job is to do what is best for U.S. national security and the American people. That is what we have done in Syria and what we will continue to do.”

Roebuck, a former ambassador to Bahrain, has been in northern Syria for nearly two years, working with the Syrian Kurds and Arab military. Arabs and Kurds in the region have a bloody history.

“Turkey’s military operation in northern Syria, spearheaded by armed Islamist groups on its payroll, represents an intention-laced effort at ethnic cleansing,” Roebuck wrote, characterizing the abuses as “war crimes.”

Roebuck said some 200 civilians have been killed and at least 100,000 displaced by the “unprovoked military operation.”

Amnesty International characterized the attacks as “horrific” and said there is “no justification for indiscriminately shelling civilian areas using imprecise weapons such as mortars.”

Though Roebuck lauded Trump’s decision to deploy troops to guard oil fields in the region to prevent them from falling into the hands of ISIS terrorists, he said the “protection of oil” rationale “plays into toxic Middle Eastern conspiracy theories.”