National Security Adviser Herbert Raymond McMaster is the newest member of Washington’s fastest-growing social club: former Trump White House officials.

McMaster, a Lieutenant General in the United States Army and a veteran of three wars, is the latest in a recent string of casualties in an administration that has seen the departure of numerous senior officials—a purge that comes a mere five months after Trump mused that his cabinet amounted to “one of the finest group of people ever assembled.”

“I am pleased to announce that, effective 4/9/18, @AmbJohnBolton will be my new National Security Advisor,” President Donald Trump tweeted on Thursday. “I am very thankful for the service of General H.R. McMaster who has done an outstanding job & will always remain my friend. There will be an official contact handover on 4/9.”

Shortly after the president announced that he would be replaced by former U.N. ambassador and noted hyper-interventionist John Bolton, McMaster announced that he plans to retire from the U.S. Army this summer.

“After thirty-four years of service to our nation, I am requesting retirement from the U.S. Army effective this summer after which I will leave public service,” McMaster said in a statement, noting that he was “thankful” Trump for his time as national security adviser.

McMaster is the president’s second national security adviser—the first, Michael Flynn, resigned after a mere 24 days following revelations regarding undisclosed contact he had made with Russian government officials and has since pleaded guilty to perjury. His exit follows months of tensions between McMaster and the president. McMaster had been a vocal advocate on behalf of the Iran nuclear deal, a lonely position to take in the Trump administration.

McMaster had been rumored to be on the outs with President Trump for months, most recently due to public comments calling evidence of Russian interference in the U.S. presidential election “incontrovertible.” The remarks prompted a swift reaction from the Trump on Twitter:

But a primary obstacle to McMaster’s continued employment was reportedly his staunch defense of the Iranian nuclear deal, which directly conflicted with Trump’s promises, extending back to the presidential campaign, to kill the agreement. Rex Tillerson, who Trump fired via Twitter on March 13, was another proponent of staying in the deal, which lifted sanctions on Iran in exchange for imposing limitations on the nation’s nuclear ambitions.

Trump had campaigned against the Iran deal, telling pro-Israel lobbying group AIPAC that his “number one priority” as president would be “to dismantle the disastrous deal with Iran,” but as national security adviser, McMaster met with top senators on the Senate Foreign Relations Committee in January to help protect the agreement.

McMaster’s public disagreements with the president and his staff on the Iran deal and Russia might have been anticipated, had President Trump read McMaster’s Ph.D. dissertation. Dereliction of Duty: Lyndon Johnson, Robert McNamara, The Joint Chiefs of Staff, and the Lies that Led to Vietnam, which was eventually published as a popular book, criticized the Joint Chiefs of Staff for deferring to President Lyndon Johnson’s wishes regarding U.S. strategy in Vietnam rather than provide counsel that conflicted with the president’s ego.

“Through their own actions,” McMaster wrote in the book, “the Chiefs… always found themselves in the difficult position of questioning a policy that the president had already approved.”