In 2015, President Obama named McGurk the presidential envoy to the global coalition against the Islamic State, and he stayed in that role under Trump. The veteran diplomat had planned to leave his post in mid-February, according to the Associated Press, but he expected U.S. direct engagement to continue for at least several more months. “Nobody is declaring a mission accomplished,” he told reporters at a State Department briefing on December 11. “It would be reckless if we were just to say, ‘Well, the physical caliphate is defeated, so we can just leave now.’”

Barely a week later came Trump’s withdrawal announcement, quickly followed by McGurk’s resignation, effective December 31. He joined a group of foreign-policy experts at Stanford University’s Freeman Spogli Institute for International Studies. Former Secretary of State Condoleezza Rice, one of his new colleagues, called him “the consummate professional diplomat,” which sounded like an endorsement of his resignation and a subtle rebuke of Trump’s abrupt announcement.

Trump took to Twitter to attack McGurk, pointing out that the diplomat was an Obama appointee and bashing him as a “grandstander” since he had simply moved up his exit by six weeks. The president added that he did not know McGurk, his own point man in the fight against the Islamic State.

Read: The U.S. isn’t really leaving Syria and Afghanistan

Now McGurk is taking his case to the American public. He says that even with a slightly elongated withdrawal timetable, Trump’s decision has damaged U.S. strategic interests and national security.

“Only Russia and Iran hailed Trump’s decision,” McGurk wrote in a Washington Post op-ed that appeared in Sunday’s print edition. “Whatever leverage we may have had with these two adversaries in Syria diminished once Trump said we would leave.” The diplomat wrote that America’s strategic rivals now face little constraint on their military buildup in Syria. Israel, the United States’ closest ally in the region, must step up air strikes to fend off Iranian threats along its northeastern border.

Safety is more assured, though, for Bashar al-Assad because “without us, any chance of upending this mass-murdering dictator, propped up by Iran and Russia, is a pipe dream,” McGurk wrote. U.S. partners are reopening embassies in Damascus and moving closer to Assad, hoping to “dilute Russian, Iranian and Turkish influence in Syria.”

The expected power vacuum may come back to haunt the United States, much as it did after the withdrawal from Iraq, McGurk forecasted. “The Islamic State and other extremist groups will fill the void opened by our departure, regenerating their capacity to threaten our friends in Europe—as they did throughout 2016—and ultimately our own homeland,” he wrote. While defeating the terror group was Trump’s professed goal, the former diplomat said, “his recent choices, unfortunately, are already giving the Islamic State—and other American adversaries—new life.”