The Pentagon's probe marks an extraordinary escalation in the Trump administration's backlash against six Democrats with military or intelligence service backgrounds who made the viral video.

Kelly -- a decorated Navy combat pilot and former astronaut who commanded the Space Shuttle Endeavour's final flight -- fired back that he would not be intimidated or "silenced by bullies."

The Pentagon had threatened to recall the Arizona senator back to active duty in order to face a court-martial under the Uniform Code of Military Justice.

Officials indicated that Kelly may have undermined the "loyalty, morale, or good order and discipline of the armed forces," citing laws barring interference in military cohesion.

It is highly unusual for the Pentagon -- long allergic to overt politics until President Donald Trump's return to power this year -- to publicly threaten a sitting member of Congress.

The video posted on Friday called on the military to "refuse illegal orders" and featured Kelly alongside Michigan Senator Elissa Slotkin and four members of the US House of Representatives.

Trump initially accused the group of "seditious behavior, punishable by death."

Over the weekend, he wrote in an all-caps social media rant that the "traitors" who told troops to disobey him "should be in jail."

- 'Acting like fascists' -

The Democrats did not specify which orders they meant, but Trump has ordered the National Guard into multiple US cities -- often against local objections -- to curb what he calls rampant unrest.

Overseas, Trump has ordered strikes on alleged drug-smuggling vessels in the Caribbean and eastern Pacific Ocean that killed more than 80 people and which experts say are illegal.

Defense Secretary Pete Hegseth branded the Democrats the "Seditious Six" and called their video "despicable, reckless, and false."

He accused Kelly of bringing "discredit" on the armed forces, saying his remarks were addressed directly at troops while invoking his rank -- giving them an air of authority.

But analysts and Kelly's supporters warned the move could backfire by elevating him ahead of a potential 2028 presidential run.

Kelly posted a photo of his many military medals, and recapped his Navy and NASA career.

"If this is meant to intimidate me and other members of Congress from doing our jobs and holding this administration accountable, it won't work," he said.

"I've given too much to this country to be silenced by bullies who care more about their own power than protecting the Constitution."

In a blistering, expletive-laced video, fellow Arizona senator Ruben Gallego, an Iraq War veteran, called the Pentagon announcement "insane" and suggested Hegseth and others were "acting like fascists."

"Mark Kelly is a patriot. There's no reason why they're going after him," he said. "He was doing his duty and just reminding people about their rights as service members."