THE PENTAGON -- Steve Bannon is not the only target of the president's ire. On Tuesday night, President Trump mocked Kim Jong Un and North Korea's nuclear capabilities in an extraordinary tweet.

No American president has ever made such a blatant nuclear threat. Responding to Kim's boast that he has a nuclear button on his desk, Mr. Trump tweeted: "I too have a nuclear button that is bigger and more powerful ... and my button works."

North Korean Leader Kim Jong Un just stated that the “Nuclear Button is on his desk at all times.” Will someone from his depleted and food starved regime please inform him that I too have a Nuclear Button, but it is a much bigger & more powerful one than his, and my Button works! — Donald J. Trump (@realDonaldTrump) January 3, 2018

The tweet unsettled former Vice President Biden, who once had the authority to hit the button if something happened to President Barack Obama.

"This is not the stuff to be tweeting about," Biden said. "It is a very, very, very difficult problem."

The device used to launch a strike is not an actual button, but a briefcase called the "football," which is carried by an aide who follows the president everywhere. It contains the codes the commander-in-chief would need to order the use of nuclear weapons.

Whatever you call it, American nuclear forces are incomparably more powerful than North Korea's. One U.S. submarine carries more warheads than Un has in his entire arsenal.

So, the president's tweet is not an empty boast. But would he really use nuclear weapons against North Korea?

"I think it's a throwaway remark that is careless, but I don't think it signals a change in nuclear policy," said former Sen. Sam Nunn, who is founder of an institute attempting to reduce the risk posed by weapons of mass destruction.

American policy has long been to use nuclear weapons only in extreme circumstances, such as a nuclear attack against the U.S. Nuclear war remains all but unthinkable, and according to Nunn, making threats about it should be unspeakable.

"I think it increases the risk of miscalculation, or what I call war by blunder," Nunn said. "To me, that's the greatest danger here, and the heated rhetoric on both sides creates a whole lot more risk in that regard. We could easily have a war nobody intended and nobody wanted."

If it comes to war, the U.S. should be able to destroy the North Korean regime without resorting to nuclear weapons, which could spread radioactive fallout over allies like Japan and South Korea, not to mention China and Russia.