The hearing was called by the Republican Chairman of the Senate Foreign Relations Committee, Senator Bob Corker of Tennessee. Sen.
"In the past" Feaver said, "Congress has played a vital role in pushing the Executive Branch to strengthen the nuclear command and control system, and the time may be ripe for another close look".
He quickly followed that up by saying "I fear that in the age of Trump, the cooler heads and strategic doctrine that we once relied upon as our last best hope against the unthinkable seem less reassuring than ever".
"I don't think that the assurances that I've received today will be satisfying to the American people", said Markey, a Democrat from MA.
The president and his top officials have said repeatedly that North Korea would not be allowed to threaten the U.S. with nuclear weapons, but as Pyongyang has persisted with its nuclear and missile tests, it has been unclear what the administration would do to stop the regime.
Other former national security officials testified that if there isn't an imminent attack, it would be more hard for the president to launch a nuclear attack out of the blue.
Rubio was referring to North Korean dictator Kim Jong Un ― but his description was similar to the way Democrats on the panel were describing Trump.
Opening the hearing, Corker - who has recently been engaged in bitter exchanges with Trump over his fitness for office - noted that "the president has the sole authority to give that order, whether we are responding to a nuclear attack or not".
Trump has countered by calling Corker a "lightweight" who couldn't get re-elected if he ran again in 2018.
"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. "It's going to be a very robust period of time".
"It should be the congressional prerogative to declare nuclear war", added Markey, who has written a bill to ban the president from being able to launch a first nuclear strike against North Korea without the authorization of Congress.
If there is a sense that Trump rashly ordered a nuclear launch, multiple sources tell CNN that senior generals would explain the risk of such a move.
Senators trying to prevent President Donald Trump from launching an unprovoked nuclear attack were stymied Tuesday, after a panel of experts warned them against rewriting laws to restrain a commander in chief many worry is impulsive and unpredictable enough to start a devastating worldwide crisis. In other words, there is time to debate strike options. In developing options for North Korea, the Pentagon has a package of what it calls flexible response options, which includes ensuring USA aircraft and missile launching ships are always in place if needed. In this scenario, Mattis, national security adviser Lt. Gen. H.R. McMaster and Secretary of State Rex Tillerson are not part of the chain of command.
Retired Gen. Robert Kehler, who previously headed the U.S.'s command that would be in charge of the nuclear arsenal during a war, said while the USA military is obligated to follow legal orders, it is not duty bound to adhere to illegal ones. To do otherwise would be considered treasonous.
Protesters at the US Senate Foreign Relations Committee hearing.
The panel ultimately, however, appeared to side against reining in the president's power to exercise nuclear authority.
"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.
"Taking away the president's authority as commander in chief or diluting it in some respect by requiring him to go to another constitutional officer in a formal sense, I'm not sure that is a wise course", McKeon said. "It's not the only tool in the toolkit to try to address something like that", Mattis said. In recent months, lawmakers have insisted the president seek Congress's approval before revoking any sanctions against Russian Federation, and momentum is building for a new authorization for use of military force (AUMF) to address the military's current and future operations against the Islamic State and other extremist groups.
"I think that we have to keep trust, keep faith in the system that we have that has proven effective now for decades", he said.