Secretary of Defense Chuck Hagel unexpectedly submitted his resignation on Monday after a series of recent discussions with President Obama and White House officials determining “that it was time for new leadership at the Pentagon,” said a senior defense official, speaking anonymously.

Hagel will remain in his role until the White House appoints a replacement, who must also be confirmed by the Senate. According to the Washington Post, potential replacements include Michele Flournoy and Ashton Carter, former high-ranking defense officials from Obama’s first term.