Gen. H.R. McMaster resigning as national security adviser

President Donald Trump and National Security Adviser Gen. H.R. McMaster mutually agreed that the three-star general and Iraq war veteran will leave the Trump administration, the White House confirmed on Thursday.

He will be replaced by former United Nations ambassador John Bolton — the third person to take on the role during the current administration.

“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. Throughout my career it has been my greatest privilege to serve alongside extraordinary servicemembers and dedicated civilians," McMaster said in a statement adding. "I am thankful to President Donald J. Trump for the opportunity to serve him and our nation as national security advisor.”

Trump lauded McMaster's service saying "I thank General McMaster and his family for their service and wish them the very best.”

McMaster's departure in early 2018 had been planned and expected, especially as Trump re-tools his team ahead of historic talks with North Korea, sources with direct knowledge told ABC News.

He was seen as adding a steady and intellectual voice to Trump’s security team, but the president chafed at his style and disposition in Oval Office briefings, the sources said. He also clashed with former Trump advisor Steve Bannon and other top military brass, including Defense Secretary Jim Mattis.

He was also reportedly unable to keep the president's attention in complex briefings, correcting him when he doesn't want to be corrected, and seen as a lecturer, the sources said.

Three months on the job, the New York Times reported that Trump described McMaster as "a pain" and complained he talked too much.

Just last month, following new indictments targeting Russian cyber operatives, Trump publicly rebuked McMaster, tweeting the general forgot to mention that results of the 2016 election were not impacted or changed by the Russians - something the indictment never said.

McMaster also broke with Trump on several issues.

He has urged continuation of the Iran nuclear deal; opposes the label “radical Islamic terrorism;” supports free trade deals with key allies like South Korea, and forcefully protested the racially charged violence in Charlottesville.

After reports surfaced that Trump revealed highly classified info to the Russians in an Oval Office meeting in May 2017, McMaster vigorously defended the president in an ad hoc press conference outside the West Wing.

He remains on active duty as a Lieutenant General in the US Army (3-star). The Pentagon has considered giving McMaster a fourth star and new assignment in the military.

McMaster's resignation comes on the heels of other high profile departures including deputy national security adviser Dina Powell, staff secretary Rob Porter, and senior adviser Hope Hicks.

By appointing John Bolton to the role of national security adviser, President Trump has just inserted one of Washington's most vocal war hawks to work beside him in the West Wing.

This worries some congressional Democrats who took to social media to decry Bolton's appointment.

The move comes less than a week the president fired Rex Tillerson, whom he had clashed with over their approach to confronting Iran.

Bolton does not mince words on Iran.

“Our goal should be regime change in Iran," he said earlier this year.

He has advocated for preemptive strikes in both Iran and North Korea.

Just last month he wrote an article in the Wall Street Journal making the case for striking North Korea, and in March of 2015, he made a similar case for Iran in an editorial in The New York Times.

With McMaster and Tillerson out and Bolton in, the future of the Iran deal seems more questionable. Trump has until May 12 to decide if he will again sign sanctions waivers.