Senate committee considers Trump's authority to launch nuclear weapons Witnesses at the hearing stressed that there are some checks.

 -- For the first time in 40 years, a Senate committee reviewed the president’s singular authority to launch nuclear weapons — a move which comes amid increased tensions with North Korea.

The Senate Foreign Relations Committee hearing was chaired by Sen. Bob Corker, R-Tenn., a frequent critic of President Donald Trump’s foreign policy approach and rhetoric. Though he said just a month ago that President Trump could be leading the U.S. “ on the path to World War III”, Corker told reporters outside the meeting this morning that the hearing was “not in any way” a rebuke of the president.

President Trump, who wrapped up a 13-day trip to Asia, previously called Pyongyang’s nuclear ambitions a global threat and once warned Kim Jong Un that if North Korea threatened the U.S., America has "military solutions" "locked and loaded”. During his recent trip to the region, however, Trump dialed back the rhetoric and said he hoped North Korea would “come to the table and make a deal” and back down from its nuclear aims.

During Monday’s hearing, Gen. C. Robert Kehler, former commander of Strategic Command, emphasized that a presidential order to use a nuclear weapon must be legal. The basic legal principles of proportionality and necessity apply to the use of nuclear weapons, he said, and "if the order was considered to be illegal, the military is obligated to refuse to follow it."

“The U.S. military doesn’t blindly follow orders,” Kehler said in his opening remarks.

Democratic Senators were, nonetheless, concerned about the president’s previous remarks.

Sen. Ben Cardin, D-Md. quoted Trump’s “fire and fury” comments and threats to “totally destroy” North Korea, saying many interpret the sharp rhetoric “to mean that the president is actively considering the use of nuclear weapons in order to deal with the threat of North Korea. That is frightening. And as the chairman pointed out, based on my understanding of the nuclear command and control protocols, there are no checks – no checks – on the president’s authority.”

Witnesses at the hearing stressed that there are some checks. The system requires the president to work with military aides and give orders that must be followed down a chain of command.

The witnesses described teams of legal advisors working for the Commander of Strategic Command General John Hyten and in the office of the Secretary of Defense James Mattis who would advise the chain of command on the legality of a hypothetical order to use nuclear weapons.

Peter Feaver, professor of political science and public policy at Duke University, reminded the committee that “the system is not a button the president can accidentally lean on, against, on the desk and immediately cause nuclear missiles to fly.”

On Monday, Mattis was directly asked about the hearing and its implications.

“I’m the president’s principal adviser on the use of force,” he told reporters, confirming he is comfortable with the way the system works.

The committee’s Democrats also expressed concerns about whether the military could refuse to obey an order to launch nuclear weapons.

Cardin doubled down on his questioning of Kehler asking, “If you believe that this did not meet the legal test of proportionality, even if ordered by the president of the United States to use a nuclear first strike, you believe that because of legalities you retain that decision to disobey?”

Kehler responded “Yes, if there is an illegal order presented to military, the military is obligated to refuse to follow it.”

Sen. Edward Markey, D-Mass., still likened the process to simple technology.

“I don’t think the assurances I’ve received today will be satisfying to the American people, I think they can still realize that Donald Trump can launch nuclear codes just as easily as he can use his Twitter account without a check and balance.”

ABC News’s Ali Rogin and Luis Martinez contributed to this report.