U.S. Strategic Command (Stratcom) has the obligation to refuse to launch nuclear weapons if an order to do so is illegal, the former head of the command said Tuesday.

But retired Gen. C. Robert Kehler conceded that he does not what would happen after Stratcom's head raises questions of legality, saying the scenarios presented by senators have not happened.

“I would have said, ‘I have a question about this' and I would have said 'I’m not ready to proceed,’” Kehler told the Senate Foreign Relations Committee when asked what he would have done if he felt an order to launch nuclear weapons had not been thoroughly vetted.

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But asked by Sen. Ron Johnson Ronald (Ron) Harold JohnsonThe Hill's Morning Report - Sponsored by The Air Line Pilots Association - White House moves closer to Pelosi on virus relief bill Second GOP senator to quarantine after exposure to coronavirus GOP-led panel to hear from former official who said Burisma was not a factor in US policy MORE (R-Wis.) what would happen next, Kehler said, “I don’t know exactly."

"Fortunately ... these are all hypothetical scenarios.”

Kehler was testifying alongside other former officials at the first House or Senate Foreign Relations committee hearing on the nuclear command and control structure since 1976.

The hearing, convened by the Senate committee's chairman, Sen. Bob Corker Robert (Bob) Phillips CorkerHas Congress captured Russia policy? Tennessee primary battle turns nasty for Republicans Cheney clashes with Trump MORE (R-Tenn.), has been seen as rebuke of President Trump, given Corker’s fierce criticism of the president in recent weeks.

But Corker asserted before the hearing that, “this is not specific to anybody."

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“To be clear, I would not support changes that would reduce our deterrence of adversaries or reassurance of our allies,” Corker, who announced his retirement in September, said. “But I would like to explore, as my predecessors in the House did 41 years ago, the realities of this system.”

Still, senators drilled down on what would happen if a president were to call the military out of the blue to order a nuclear strike, with some Democrats explicitly pointing to Trump.

“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 weapons strike that is wildly out of step with U.S. national security interests,” Sen. Chris Murphy Christopher (Chris) Scott MurphyDemocratic senator calls for 'more flexible' medical supply chain to counter pandemics The Hill's 12:30 Report - Presented by Facebook - Don't expect a government check anytime soon GOP chairman to release interim report on Biden probe 'in about a week' MORE (D-Conn.) said during the hearing. “So let’s just recognize the exceptional nature of this moment.”

Kehler said that an order to launch nuclear weapons has to meet the same legal threshold as any other military order, meaning it has to be proportional, necessary and distinguish between combatants and civilians.

“The military does not blindly follow orders,” he said.

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But the process for the military to determine whether an order is illegal is unclear, he added.

“If there is an illegal order presented to the military, the military is obligated to refuse to follow it,” Kehler said in response to Sen. Ben Cardin Benjamin (Ben) Louis CardinPPP application window closes after coronavirus talks deadlock Congress eyes tighter restrictions on next round of small business help Senate passes extension of application deadline for PPP small-business loans MORE (D-Md.). “Now the question is just the one that you’ve described, is the process leading to that determination and how you arrive at that. And I would concede to you that would be a very difficult process and a very difficult conversation. But in this scenario that you’re painting, I would also argue that there’s time for that.”

Peter Feaver, a professor at Duke University who served in the George W. Bush and Clinton administrations, said “there would be a lot of people” determining the legal authority to launch a nuclear strike.

“It wouldn’t be the president alone persuading a single military officer alone on the other side of the telephone,” he said. "There would be a large group of advisers and legal advisers weighing in on this.”

Brian McKeon, a senior national security official in the Obama administration, added later that it’s up to senior officers and the secretary of Defense to push back on an illegal order, not the officers carrying out the nuclear strike.

“As Gen. Kehler has described, the officers in the chain of command, the senior officers and the secretary could raise objections if they believe the order is illegal,” McKeon said. “I think the system is designed to protect the first or second lieutenant, the 23-year-old Air Force officer sitting in a launch control center, from having to make that grave decision. It’s really the four-stars and the secretary who need to bear that burden.”