National Security Adviser H.R. McMaster has been replaced by former Ambassador to the United Nations John Bolton as national security adviser, President Trump tweeted Thursday. That transition will be effective April 9.

"I am pleased to announce that, effective 4/9/18, @AmbJohnBolton will be my new National Security Advisor," Mr. Trump wrote. "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." 

Bolton was spotted at the White House earlier in the day, fueling speculation. Bolton will be Mr. Trump's third national security adviser.

McMaster's departure was essentially reported to have been a fait accompli by last week by CBS News chief White House correspondent Major Garrett. Like ousted Secretary of State Rex Tillerson, McMaster had never really developed a strong relationship with the president. Bolton is known to be a hardliner on the Iran nuclear deal and on North Korea. Bolton's arrival makes it far more likely Mr. Trump will withdraw from the Iran deal soon. 

The major staff change comes at a pivotal point in foreign relations, as the U.S. is looking to meet with North Korea. The White House has suggested the president intends to meet North Korean leader Kim Jong Un by the end of May. Three weeks ago, Bolton authored a piece in the Wall Street Journal, arguing the legal case for a preemptive strike on North Korea. 

In a statement released by the White House, Mr. Trump thanked McMaster for his service and praised his decades of military service. McMaster, who is an active duty three-star general, said in a statement that after 34 years of service, he would be requesting retirement from the U.S. Army, effective this summer." 

This is a developing story and will be updated. 

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