Here's a question rarely raised before Donald Trump ran for the White House: If the president ordered a pre-emptive nuclear strike, could anyone stop him?

The answer is no.

Not the Congress. Not his secretary of defense. And by design, not the military officers who would be duty-bound to execute the order.

Get Breaking News Delivered to Your Inbox

As Bruce Blair, a former nuclear missile launch officer and expert on nuclear command and control, has put it, "The protocol for ordering the use of nuclear weapons endows every president with civilization-ending power." Trump, he wrote in a Washington Post column last summer, "has unchecked authority to order a preventive nuclear strike against any nation he wants with a single verbal direction to the Pentagon war room."

Or, as then-Vice President Dick Cheney explained in December 2008, the president "could launch a kind of devastating attack the world's never seen. He doesn't have to check with anybody. He doesn't have to call the Congress. He doesn't have to check with the courts."

And the world has changed even more in the decade since, with North Korea posing a bigger and more immediate nuclear threat than had seemed possible. The nature of the U.S. political world has changed, too, and Trump's opponents — even within his own party — question whether he has too much power over nuclear weapons.

These realities will converge Tuesday in a Senate hearing room where the Foreign Relations Committee — headed by one of Trump's strongest Republican critics, Sen. Bob Corker of Tennessee — will hear testimony from a former commander of the Pentagon's nuclear war fighting command and other witnesses. The topic: "Authority to order the use of nuclear weapons."

Corker said numerous lawmakers have raised questions about legislative and presidential war-making authorities and the use of America's nuclear arsenal.

"This discussion is long overdue," Corker said in announcing the hearing.

Alex Wellerstein, a historian of science at the Stevens Institute of Technology who has researched and written extensively about presidential nuclear authority, said he hopes the discussion "might shed some more light on aspects of the procedures for presidential use of nuclear weapons that I think really needs to be known and talked about."

He said the U.S. system has evolved through tradition and precedent more than by laws.

"The technology of the bomb itself does not compel this sort of arrangement," he wrote in an email exchange. "This is a product of circumstances. I think the circumstances under which the system was created, and the world we now live in, are sufficiently different that we could, and perhaps should, contemplate revision of the system."

Asked about this Monday in an impromptu exchange at the Pentagon, Defense Secretary Jim Mattis was reluctant to describe his role in nuclear strike decision-making. "I'm the president's principal adviser on the use of force," he said. Asked whether he was comfortable with the system as it exists, he said, "I am," but did not elaborate.

Some aspects of presidential nuclear war-making powers are secret and therefore not well understood by the public. The system is built for fast decision-making, not debate. That's because speed is seen as essential in a crisis with a nuclear peer like Russia. Unlike North Korea, Russia has enough nuclear weapons to destroy the U.S. in minutes.

Russia's long-range missiles could reach the U.S. in about 30 minutes. Submarine-launched missiles fired from nearer U.S. shores might arrive in half that time. Given that some of the U.S. response time would be taken up by administrative steps, the president would have less than 10 minutes to absorb the information, review his options and make his decision, according to a December 2016 report by nuclear arms specialist Amy Woolf of the Congressional Research Service.

A president who decided to launch a nuclear attack — either in retaliation for a nuclear strike or in anticipation of one — would first hold an emergency conference with the defense secretary, the Joint Chiefs of Staff chairman and other advisers. The commander of U.S. Strategic Command, now Air Force Gen. John Hyten, would brief the president on strike options, and the president would make his decision.

The president would communicate his decision and transmit his authorization through a device called the nuclear football, a suitcase carried by a military aide. It's equipped with communication tools and a book with prepared war plans.

If the president decided to order a strike, he would identify himself to military officials at the Pentagon with codes unique to him. These codes are recorded on a card known as the biscuit that is carried by the president at all times. He would then transmit the launch order to the Pentagon and Strategic Command.

Blair, the former missile launch officer, said there is no way to reverse the president's order. And there would be no recalling missiles once launched.

Although fielded and assigned for use by the military, the nuclear bomb is inherently a political weapon, given its almost unimaginable destructive capacity. That explains why the system for controlling the use of U.S. nuclear weapons has been designed to concentrate decision-making power in the ultimate political office: the presidency.