Sen. Murphy: Trump So 'Unstable' He May Order Nuclear Strike

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"Nothing happens automatically". A nuclear first strike would need to meet certain legal requirements, he added, noting that the military is obligated to disobey an "illegal order".

The assurances came at the first congressional hearings since 1976 on presidential authority to order the use of the U.S. nuclear arsenal, against a background of mounting concern over North Korea's nuclear programme - and Donald Trump's emotional stability.

Senate Foreign Relations Committee chairman Bob Corker, R-Tenn., who has said Trump's threats to global rivals could put the country "on the path to World War III," began Tuesday's session warning of the inherent danger in a system where the president has "sole authority" to give launch orders there are "no way to revoke".

Marco Rubio, who ran against Trump in the Republican presidential primary previous year, was among lawmakers who were quick to point out that the hearing should not be taken as a reduced USA nuclear posture.

The Senate Foreign Relations Committee convened to examine presidential authority to order use of nuclear weapons. "If there is an illegal order presented to the military, the military is obligated to refuse to follow it".

"We are concerned that the president of the United States is so unstable, is so volatile, has a decision-making process that is so quixotic that he might order a nuclear weapons strike that is wildly out of the step with USA national security interests", Murphy said.

"Taking away the president's authority as commander in chief or diluting it in some respect by requiring him to go to another constitutional officer in a formal sense, I'm not sure that is a wise course", McKeon said.

One senator, Ron Johnson, a Republican from Wisconsin, asked: "Then what happens?"

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"I don't know", Kehler admitted, to nervous chuckles in the chamber. "The human factor kicks in". Indeed, a military aide shadows the commander in chief day and night, carrying the black briefcase commonly referred to as the "nuclear football", packed with attack options and other information needed in a national emergency. The question is the process leading to that determination and how you arrive at that.

Kehler acknowledged, however, should the military reject a nuclear strike order that would result in a "very hard conversation".

"While it's true that military personnel have an obligation to disobey illegal or immoral orders, how will the "operators" at the bottom of the chain of command - the young men and women tasked with flying our nuclear bombers and launching our deadly land- and sea-based ballistic missiles - determine a president's order to use those weapons is unjustifiable, especially when they are expected to execute those orders in just a few minutes and train constantly to do so?" he said. "That's a very thin reed on which to have the fate of the planet being dependent". "I think they can still realize that Donald Trump can launch nuclear codes just as easily as he can use his Twitter account".

The escalating war of words has alarmed U.S. lawmakers.

"It should be the congressional prerogative to declare nuclear war", added Markey, who has written a bill to ban the president from being able to launch a first nuclear strike against North Korea without the authorization of Congress.

Addressing the committee, Brian McKeon, a former top Pentagon official, said Trump's tweets during the ongoing stand-off with North Korea should not be taken lightly given that North Korea does not have the capabilities to check to see if some of Trump's comments are mere bluster.

Peter Feaver, a politics professor at Duke University and a specialist on presidential war powers, said: "I would say distinguish between scenarios where the military wake up the president versus scenarios where the presidents wake up the military". "I don't think the assurances I've received today will be satisfying to the American people. And they would be asking the questions that would slow down that process".