Mattis was asked whether the president could launch a first strike with nuclear weapons, without consulting Congress, against another nuclear-armed country preparing to attack the U.S.
No Trump administration officials are testifying before the hearing, which is examining the nuclear command and control structure that has served all United States presidents. "I don't think the assurances I've received today will be satisfying to the American people".
In August, Trump threatened North Korea with "fire and fury like the world has never seen".
"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 strike that is wildly out of step with USA interests", said Sen. Corker has become an outspoken critic of the president, although he says Tuesday's hearing was not specific to President Trump.
The current process for deploying nuclear weapons "means that the president has the sole authority to give that order - whether we are responding to a nuclear attack or not", said Corker, who has vowed a series of hearings on the issue.
"It has implications for the deterrent, it has implications for the extended deterrent, .it has implications for our own military men and women", said retired Gen. Robert Kehler, the commander of U.S. Strategic Command from 2011 to 2013.
Military experts testifying before the committee noted that, while presidents have ultimate authority to order nuclear strikes, there are safeguards in place to ensure those orders are considered first.
The decision to launch a nuclear attack is made by the president, relayed to the nation's top uniformed military officer, known as Chairman of the Joint Chiefs of Staff, before it sent down the chain of command, according to military documents.
Sen. Edward Markey, D-Mass., still likened the process to simple technology. 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.
Democratic Senators were, nonetheless, concerned about the president's previous remarks.
Legal and military experts told lawmakers Tuesday the president does not have complete authority to launch the first strike using a nuclear weapon when the U.S.is not faced with an imminent threat.
"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 Tennessee Republican, who said he won't seek re-election next year, has had a public feud with Trump, calling the White House an "adult day care center" and saying the US secretaries of state and defense are "the people that help separate our country from chaos". Corker has since been working with Sens. It does not have bipartisan support, however, and is unlikely to pass.
Much of the nuclear chain of command is already "devoted to safety and security measures created to minimize these risks" Feaver said.
"There are legitimate disputes when it comes to the power of the president and the power of Congress", Sen.
Retired Air Force Gen. Robert Kehler told the Senate Foreign Relations Committee on Tuesday the us armed forces are obligated to follow legal orders, not illegal ones.
Democrats argued Trump is already confusing North Korea about the United States' intentions through his tweets.
"I would be very concerned about a miscalculation based on continuing use of his Twitter account", McKeon answered.