The committee's chairman, Senator Bob Corker, a Republican, warned that "once that order is given and verified, there is no way to revoke it".
Corker has become a vocal critic of President Donald Trump and referred to the White House as an "adult day-care center" in a Tweet.
Still, senators drilled down on what would happen if a president were to call the military out of the blue to order a nuclear strike, with some Democrats explicitly pointing to Trump.
Chris Murphy, Democratic senator from Connecticut, said: "We are concerned that the president of the United Status 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 step with US national security interests".
"This is not a hypothetical question", Mr Cardin said, noting that a nuclear first strike on North Korea could be an alternative to a conventional military campaign that would produce mass casualties in Japan and South Korea.
"The system is not a button that the president can accidentally lean against on the desk and immediately cause missiles to fly, as some people in the public, I think, fear it would be", said Peter Feaver, a Duke University political science professor.
"If there is an illegal order presented to the military, the military is obligated to refuse to follow it. The question is the process leading to that determination and how you arrive at that".
And dare I say, that if the special counsel Robert Mueller should find that crimes were committed by the Trump administration related to its its Russian connections - something we don't expect to know for many months - a House controlled by Democrats, would have the power to impeach. Military leaders could refuse to follow any order that they deemed illegal or that had not been vetted through the proper channels, Kehler said. "That's a very thin reed on which to have the fate of the planet being dependent".
'That would be a very difficult process, and a very difficult conversation, ' he added.
Protesters at the US Senate Foreign Relations Committee hearing
The US is closely monitoring the spread of extremism in the Southeast Asia and the growing threat from terrorist groups, including those affiliated with ISIS, he said. "This is not specific to anybody", he said.
Mr McKeon, the former Department of Defense official, echoed that view.
"I do not see a legislative solution today, but that doesn't mean that over the course of the next several months one might develop", he told reporters after the hearing.
But they acknowledged that the President could overrule the advice of his advisers and order a nuclear strike if it is deemed lawful.
"The United States is committed to engaging with nations across the Indo-Pacific region in a way that affirms their sovereignty, maintains their independence, promotes self-reliance, and advances their prosperity", he said.
'He alone would have the authority to make the decision, and I think we all believe that the system would carry out the order that he gave.
Trump said the US supported efforts to end the violence, to ensure accountability for atrocities committed, and to facilitate the safe and voluntary return of the refugees.
Sen. Ron Johnson, a Republican who has typically supported Trump, replied, 'So, we can have a little comfort that even though the president has the authority, there are limits to that, even within this context, when there is time?' "Unfortunately, I cannot make that assurance today". "Stop. We need to resolve these issues, or we need to address these questions, or whatever".
Under the US constitution, only Congress has the power to declare war, but the president, as commander-in-chief of the armed forces, has the authority to respond to an actual or imminent threat.
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