As President Donald Trump prepared in recent weeks to meet in person with Taliban negotiators at Camp David and with Iranian President Hassan Rouhani in New York later this month, National Security Advisor John Bolton grew increasingly frustrated. And on Monday, during a conversation between Bolton and the President, the two men reached their limit with one another.

In his 520 days as Trump’s third National Security Advisor, Bolton, a life-long hawk, had tried to steer the President toward a hard-line foreign policy. As Trump embraced the idea of meeting with two of America’s most ardent adversaries, Bolton objected increasingly vocally, according to several administration sources familiar with their discussions.

Then on Monday, Trump and Bolton spoke to try to clear the air. Bolton brought up the fact that he was left out of a meeting on the Afghanistan negotiations, a U.S. official who was briefed on the conversation tells TIME. As the discussion progressed, it began to spiral outward into Bolton’s broader questions about Trump’s willingness to meet with Iran’s president. “It was supposed to be a very, very limited,” discussion, the U.S. official says, “About how Bolton had been left out of a meeting on Afghanistan and it became a ‘Why are you meeting with Rouhani?’” conversation instead.

The two men offer different accounts of how things went from there. On Tuesday, Trump tweeted that he had asked for Bolton’s resignation on Monday evening, and had received it Tuesday morning. “I informed John Bolton last night that his services are no longer needed at the White House,” Trump wrote. Bolton later tweeted that it was he who had offered to resign Monday evening, and that Trump had accepted Tuesday morning.

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Either way, Bolton’s departure represents a turning point for the Trump presidency. A blunt, famously effective bureaucratic knife fighter, Bolton had sometimes succeeded in steering Trump towards a tougher line in some parts of the world, including against Iran. Since joining the White House in April 2018, Bolton did away with much of the National Security Council deliberation processes and, in a break with his camera-shy predecessors, stepped into an outsized public role. He used his Twitter account to issue dire warnings in order to keep the America’s adversaries off-balance. In several instances, B