The Pentagon on Monday threatened to recall U.S. Senator Mark Kelly, a retired Navy captain, to active duty status in order to prosecute him after what it described as seditious behavior by the former astronaut and decorated veteran.

Kelly, who denies any wrongdoing and who said in a statement he would not be intimidated, joined five other Democrats in Congress with backgrounds in the U.S. military and intelligence community to urge U.S. troops to refuse any illegal orders.

Kelly's 18 November video message came amid heightened concerns among Democrats, echoed privately by some U.S. military officials, that the Trump administration is violating the law by ordering the U.S. military to kill suspected drug traffickers in strikes on their vessels in Latin American waters.

The Pentagon says those strikes are justified because the drug smugglers are considered terrorists.

The Pentagon statement said it was reviewing "serious allegations of misconduct" against Kelly. While it did not say what charges Kelly could face if it took such an extraordinary step, U.S. Defense Secretary Pete Hegseth posted remarks on X accusing Kelly and the other lawmakers of sedition.

"The video made by the 'Seditious Six' was despicable, reckless, and false," Defence Secretary Pete Hegseth said on X.

"Encouraging our warriors to ignore the orders of their Commanders undermines every aspect of 'good order and discipline.'"

President Donald Trump has also accused Kelly and the other Democrats of sedition, saying in a social media post that the crime was punishable by death.

Under the Uniform Code of Military Justice (UCMJ), sedition and mutiny are among the most serious offenses and can be punishable by death.

Kelly, in a statement, said he learned of the threat from Hegseth's social media post. He detailed his public service prior to joining the Senate representing Arizona, including 39 combat missions in Operation Desert Storm and four space shuttle flights at NASA.

"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," Kelly 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."

The threat to prosecute Kelly follows a purge at the Pentagon of senior members of the U.S. military, including the chairman of the Joint Chiefs of Staff, the head of the Navy and the director of the National Security Agency.

The decision to recall and potentially prosecute Kelly could also be seen as a message to those recently dismissed officials, who have stayed silent following their removals.

Rachel VanLandingham, a former Air Force lawyer now at Southwestern Law School, said she had never seen sitting lawmakers called back to the military involuntarily, and that Kelly would have a strong legal case to get a preliminary injunction since there was no evidence of probable cause.

"He has strong legal standing to say 'Absolutely not. I'm not going to do this'," VanLandingham said.

Hegseth's remarks also could undermine any Pentagon effort to prosecute Kelly since they amounted to a clear case of undue command influence and could be used as evidence that Kelly would not be able to get a fair trial, she said.

The prosecution of Kelly would raise questions about free speech rights and the separation of powers under the U.S. Constitution.

But Brenner Fissell, a professor at Villanova University School of Law, said Kelly could be facing some legal risk.

Kelly lacks the protections of Speech and Debate Clause immunity, Fissell said, which protect lawmakers for statements they make on the House or Senate floor.

"If they're serious and they're planning on charging him with mutiny, sedition, it seems like they would definitely try to book him in a pre-trial detention if they believe it's that serious of an offense," Fissell, a former defence counsel at Guantanamo Bay Military Commissions, added.

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