President Donald Trump blasted his former chief of staff John Kelly on Thursday after the ex-top aide said Army Lt. Col. Alexander Vindman did the right thing in reporting his concerns about Trump's July 25 call with Ukraine's president.

"When I terminated John Kelly, which I couldn't do fast enough, he knew full well that he was way over his head," Trump tweeted. "Being Chief of Staff just wasn't for him. 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."

"His incredible wife, Karen, who I have a lot of respect for, once pulled me aside & said strongly that 'John respects you greatly. When we are no longer here, he will only speak well of you,''' Trump continued. "Wrong!"

Trump was responding to comments Kelly made during a 75-minute speech and question-and-answer session at a Wednesday night event before students and guests at Drew University in New Jersey, which The Atlantic reported.

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The retired Marine Corps general, who also served as Trump's Homeland Security secretary prior to taking on the job as his chief of staff, said Vindman was simply following his military training in reporting concerns about Trump's call.

That phone call, in which Trump asked Ukrainian President Volodymyr Zelenskiy to investigate former Vice President Joe Biden, his son Hunter and Democrats, led to Trump's impeachment. Last week, the Senate acquitted the president on two charges, although it was the first time in history a member of a president's own party — Sen. Mitt Romney of Utah — voted to convict.

Vindman "did exactly what we teach them to do from cradle to grave," Kelly said. "He went and told his boss what he just heard."

Vindman, who was the top Ukraine expert on the National Security Council and testified in the House impeachment inquiry, was escorted out of the White House last week. Trump later attributed Vindman's removal to the impeachment.

Kelly said Vindman was right to flag the call because it marked a huge change in U.S. policy toward Ukraine and suggested the content of that call was akin to hearing "an illegal order."

"Through the Obama administration up until that phone call, the policy of the U.S. was militarily to support Ukraine in their defensive fight against … the Russians," Kelly said, according to The Atlantic. "And so, when the president said that continued support would be based on X, that essentially changed. And that's what that guy [Vindman] was most interested in."

"We teach them, 'Don't follow an illegal order. And if you're ever given one, you'll raise it to whoever gives it to you that this is an illegal order, and then tell your boss,'" Kelly added.

Kelly, who departed the administration in late 2018, was also critical of other areas of the Trump presidency. He said Trump "tried" to get North Korean leader Kim Jong Un to give up his nuclear weapons "but it didn't work."

"I'm an optimist most of the time, but I'm also a realist, and I never did think Kim would do anything other than play us for a while, and he did that fairly effectively," Kelly said.

In announcing Kelly's impending departure in 2018, Trump told reporters: "John Kelly will be leaving — I don't know if I can say 'retiring.'''

"But, he's a great guy," Trump continued.