For the first time in over 40 years, Congress is examining a US president's authority to launch a nuclear attack.
The Senate Foreign Relations Committee hearing is titled Authority to Order the Use of Nuclear Weapons.
The chairman of the panel accused President Trump last month of setting the US "on a path to World War III".
In August, Mr Trump vowed to unleash "fire and fury like the world has never seen" on North Korea if it continued to expand its atomic weapons programme.
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The last time Congress debated this issue was over a four-day hearing in March 1976.
A nervous laugh
Senator Chris Murphy, a Connecticut Democrat, explained the reason for Tuesday's public hearing.
"We are concerned that the president 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."
Senators also wanted to know what would happen if the president ordered a nuclear strike.
Robert Kehler, an ex-commander of US Strategic Command, said that in his former role he would have followed the president's order to carry out the strike - if it were legal.
Mr Kehler said if he were uncertain about its legality, he would consult with his own advisers.
Under certain circumstances, he explained: "I would have said, 'I'm not ready to proceed.'"
One senator, Ron Johnson, a Republican from Wisconsin, asked: "Then what happens?"
Mr Kehler admitted: "I don't know."
People in the room laughed.
But the Eyes On Events's Tara McKelvey, who attended the hearing, said it was a nervous laugh.
End of Twitter post by @SenBobCorker
What else can we expect from the hearing?
No Trump administration officials are testifying before the hearing, which is examining the nuclear command and control structure that has served all US presidents.
But discussion about the highly classified process of actually launching an attack is unlikely in such a public forum.
The hearing is being closely watched, not only due to the grave nature of the topic, but also because of Mr Trump's vocal critics on the panel - some of whom come from the Republican president's own party.
Bob Corker, the Tennessee senator who chairs the committee, last month engaged in a Twitter spat with Mr Trump, likening the White House to "an adult day care center".
Another senator on the panel has drafted legislation proposing to curb the president's power to launch a nuclear attack.
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The bill by Edward Markey, a Massachusetts Democrat, would require Mr Trump to obtain a declaration of war from Congress before launching a nuclear first-strike.
Despite rallying 13 co-sponsors in the Senate, the measure has no Republican support and has gained little traction.
Can Trump launch a nuclear attack?
As commander-in-chief, the president has the sole authority to order a nuclear attack, which can be delivered either by submarine, airplane, or intercontinental ballistic missile (ICBM) - which make up the so-called "nuclear triad".
Under current rules, the US president could launch a strike by entering the codes into a device called "the football", which travels everywhere with the president.
Mr Trump is not required to consult anyone or gain consensus from any other members of government.
His top advisers, such as Defence Secretary James Mattis, Secretary of State Rex Tillerson, or national security adviser Lt Gen HR McMaster, play no role in the chain of command.
Congressional approval is required for the use of conventional military force.
But nuclear powers have remained under the president's authority since the dawn of the nuclear age.
This is because an enemy ballistic missile launched from the other side of the world could hit the US in barely 30 minutes.