UNITED STATES—National security adviser, H.R. McMaster, resigned from his position on Thursday, March 22.

“H. R. McMaster has served his country with distinction for more than 30 years,” said President Donald Trump in a statement. “He has won many battles and his bravery and toughness are legendary. General McMaster’s leadership of the National Security Council staff has helped my administration accomplish great things to bolster America’s national security.”

McMaster released a statement indicating he would be retiring from the Army this summer in addition to his resignation.

“I am thankful to President Donald J. Trump for the opportunity to serve him and our nation as national security advisor,” said McMaster. “I am grateful for the friendship and support of the members of the National Security Council who worked together to provide the President with the best options to protect and advance our national interests.”

General McMaster entered the post after Michael Flynn resigned last year. According to the New York Times, McMaster had a challenging relationship with White House Chief of Staff, General John Kelly. Reports indicated that McMaster and President Trump did not have a good rapport, with bad chemistry being displayed in the public spotlight.

At a conference in Munich, McMaster said that the evidence of Russian interference in the presidential election was irrefutable. The president responded with the following tweet:

“General McMaster forgot to say that the results of the 2016 election were not impacted or changed by the Russians and that the only Collusion was between Russia and Crooked H, the DNC and the Dems.”

President Trump has appointed John Bolton, former U.S. Ambassador to the United Nations, as the third national security adviser of his administration. Bolton is expected to take a hardline approach with North Korea and Iran.