President Trump tweeted Tuesday that he has asked national security adviser John Bolton to resign.

"I disagreed strongly with many of his suggestions, as did others in the Administration, and therefore I asked John for his resignation, which was given to me this morning. I thank John very much for his service."

What they're saying: Bolton tweeted that he offered Trump his resignation on Monday night, and that the president said they would discuss tomorrow. If Trump did not notify Bolton of his decision, it would mark yet another instance of a high-ranking administration official being fired via tweet.

Why it matters: Bolton was one of Trump's most hawkish foreign policy advisers and a key player in the administration's "maximum pressure" campaign against Iran. Bolton also had hardline views on U.S. policy toward Venezuela and North Korea, with critics often complaining to Trump that Bolton would drag him into an unwanted war.

Between the lines: 7 sources who have discussed Bolton with Trump told Axios' Jonathan Swan in July that the president says having Bolton on his team improves his bargaining position and gives him a psychological advantage over foes like Iran and North Korea.

However, the New York Times reports that tension between the 2 men — which has been present throughout Bolton tenure — reached new heights after Trump called off a planned airstrike against Iran and met with North Korean leader Kim Jong-un in the Demilitarized Zone.

Bolton has called for bombing Iran in the past and has publicly condemned North Korea's recent missile tests, which Trump has brushed off as insignificant.

Bolton has also been an ardent opponent of the U.S. negotiating a peace agreement with the Taliban, arguing that Trump could still fulfill his campaign promise of withdrawing U.S. troops from Afghanistan without a deal, per the Times. He was also, unsurprisingly, opposed to Trump's decision to invite the Taliban to Camp David.

What's next: Trump said he'll name a new national security adviser next week. He'll become the first president to have 4 national security advisers in his first term, per CNBC's Kayla Tausche.

Go deeper: A look at Trump's relationship with Bolton in better times