Waltz’s fate was sealed by his inclusion of a journalist on a sensitive Signal group chat in March. But he had been clashing with other top officials since early in the administration, including over whether to pursue military action against Iran, senior officials and Trump advisers said Friday.

While Secretary of State Marco Rubio will fill in temporarily as national security adviser, President Trump is likely to follow his own often quixotic instincts.

President Donald Trump said Thursday that he would nominate outgoing national security adviser Mike Waltz to be the U.S. ambassador to the United Nations. Waltz, a former congressman from Florida, has been on shaky ground with Trump since March after he inadvertently added a journalist to a private chat on the messaging app Signal with other top national security officials to discuss military strikes in Yemen.