US President Donald Trump on Tuesday announced via Twitter his decision to fire John Bolton from his post as national security adviser. The firing came as a result of disagreements over national security matters concerning Iran, North Korea and Afghanistan.

The president wrote on Twitter that he “informed John Bolton last night that his services are no longer needed at the White House” and that he “disagreed strongly with many of his suggestions, as did others in the Administration.”

Bolton contradicted Trump’s account of the sequence of events leading to his resignation, insisting that it came from his own initiative rather than Trump’s. Shortly after the President’s tweet, Bolton posted his own claiming that he had offered to resign himself and that Trump told him, “Let’s talk about it tomorrow.”

The two have clashed over Trump’s efforts to normalize relations with North Korea, despite the country’s continued short-range missile tests and continuation of its nuclear program. Bolton also supported a planned airstrike in Iran in retaliation for a downed American drone, which Trump called off.

Most recently, Trump and Bolton’s differences came to a head over the president’s now-canceled plans to invite Taliban leaders to Camp David to finalize a planned peace agreement between the US and the militant group.

The disagreements between the two have come to represent their varying approaches to foreign policy. Trump has periodically expressed skepticism of the US military’s overseas adventurism, in opposition to Bolton’s consistently hawkish stances toward hostile powers.

Trump said that he would pick a replacement for Bolton next week. The president’s pick for a new adviser could indicate the direction he wishes to take on foreign policy issues in advance of his 2020 reelection campaign.