Former White House National Security Advisor John Bolton criticized President Trump on Thursday after the president lashed out against his former chief of staff John Kelly.

“John Kelly is an honorable man,” Bolton wrote on Twitter. “John and I have disagreed at times, as is commonplace at senior government levels, but he has always served his country faithfully.”

Trump ripped into Kelly after the former general criticized the president on several policy issues in a speech to students at Drew University.

“When I terminated John Kelly, which I couldn’t do fast enough, he knew full well that he was [in] way over his head,” Trump wrote. “Being Chief of Staff just wasn’t for him.”

But Bolton, who has also been the target of Trump’s ire since volunteering to testify in the president’s impeachment trial, defended Kelly.

“Conservatives especially have a responsibility to reject baseless attacks upon him,” he wrote.

Kelly criticized Trump for firing Lt. Col. Alexander Vindman, for his remarks about immigrants, for his decision to intervene in Navy SEAL Eddie Gallagher’s case, as well as for Trump’s interactions with foreign dictators Vladimir Putin and Kim Jong-un.

Trump berated Kelly for failing to “keep his mouth shut.”

“He came in with a bang, went out with a whimper, but like so many X’s, he misses the action & just can’t keep his mouth shut, which he actually has a military and legal obligation to do,” Trump wrote on Twitter.