President Donald Trump on Monday said he fired Defense Secretary Mark Esper , after months of speculation that Esper was on his way out for defying the president on several occasions.

In a pair of tweets , Trump announced that Christopher Miller, the director of the National Counterterrorism Center, will serve as acting defense secretary.

Mark Esper has been terminated, the president tweeted. I would like to thank him for his service.

Miller is an Army Special Forces veteran who served in the White House and Pentagon before taking over the counterterror agency earlier this year. He supports the assertive use of U.S. power abroad against a range of targets, from terror groups like the Islamic State and al Qaeda to nations like Iran.

As Trump and his team prepare to hand over power to President-elect Joe Biden in January, national security experts say theres potential for some international chaos whether from American adversaries trying to take advantage of confusion stateside or from Trump administration hardliners trying to take parting shots at favorite targets like Iran. Israeli sources recently told Axios that they anticipate a flood of sanctions from the outgoing administration against Iran.

Miller is broadly respected as a career professional and had bipartisan support for his Senate confirmation to his current role, but some lawmakers view him as susceptible to political pressure from Trump. The presidents move against Esper indicates that he wanted someone he perceived as more loyal leading the Defense Department.