For the first time in over 40 years, Congress has examined a US president's authority to launch a nuclear attack.
The Senate Foreign Relations Committee hearing was titled Authority to Order the Use of Nuclear Weapons.
Some senators expressed concern that the president might irresponsibly order a nuclear strike; others said he must have the authority to act without meddling from lawyers.
The last time Congress debated this issue was in March 1976.
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.
Last month, the Senate committee's Republican chairman, Senator Bob Corker, accused the president of setting the US "on a path to World War III".
Senator Ben Cardin set the tone at Tuesday morning's public hearing on Capitol Hill.
"This is not a hypothetical discussion," the Maryland Democrat said.
Some senators present said they were troubled about the president's latitude to launch a nuclear strike.
Chris Murphy, a Democrat from Connecticut, said: "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."
One of the experts, C Robert Kehler, who was commander of the US Strategic Command from 2011-13, said that in his former role he would have followed the president's order to carry out the strike - if it were legal.
He said if he were uncertain about its legality, he would have consulted with his own advisers.
Under certain circumstances, he explained: "I would have said, 'I'm not ready to proceed.'"
One senator, Ron Johnson, a Wisconsin Republican, asked: "Then what happens?"
Kehler admitted: "I don't know."
People in the room laughed. But it was a nervous laugh.