U.S. military officers won’t “blindly” follow a hypothetical presidential order to launch a nuclear strike, a retired general told lawmakers Tuesday.

“It's important to remember that the United States military doesn't blindly follow orders,” C. Robert Kehler, who led the U.S. Strategic Command from 2011 to 2013, told the Senate Foreign Relations Committee. “A presidential order to employ U.S. nuclear weapons must be legal.”

Kehler, whose duties at the Nebraska-based Strategic Command rendered him directly responsible for American nuclear forces, appeared as part of a panel of witnesses testifying about the scope and limits of presidential authority over nuclear weapons. The committee convened the hearing over concerns about whether President Trump might order such an attack on North Korea without congressional approval.

“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 weapon strike that is wildly out of step with U.S. national security interests,” Sen. Chris Murphy, D-Conn., said during the hearing.

Committee Chairman Bob Corker, R-Tenn., who called the hearing, maintained a more respectful tone despite previously mocking Trump’s maturity.

“Once that order is given and verified, there is no way to revoke it,” he noted. “To be clear, I would not support changes that would reduce our deterrence of adversaries or reassurance of our allies. But I would like to explore, as our predecessors in the House did 41 years ago, the realities of this system.”

Kehler cautioned against dramatic changes to the current process. "I urge Congress to carefully consider the potential impacts to deterrence and extended deterrence that any potential changes to nuclear command and control might have,” he warned. “I also urge you to consider that conflicting signals can result in loss of confidence, confusion, or paralysis in the operating forces at a critical moment.”

The retired general emphasized that Trump has broad power to issue that order if he deems it necessary, but his comments about the legality of the mandate signaled that there are certain hedges around the decision.

“The basic legal principles of military necessity, distinction, and proportionality apply to nuclear weapons just as they do to every other weapon,” he said. “It was my job and the job of other senior leaders like the secretary of defense and the chairman [of the Joint Chiefs of Staff] and other combatant commanders to make sure these principles were applied to nuclear orders.”

The United States has not used nuclear weapons in conflict since the Second World War, so there’s no known precedent for a commander refusing to carry out a nuclear strike order. The question has arisen, though, because of North Korea’s recent nuclear weapons and ballistic missile tests — the testing of an intercontinental ballistic missile in July raised western alarms that the regime could soon have the ability to strike the mainland of the United States with a nuclear weapon.

"They are closer now than they were five years ago, and I expect they will be closer in five months than they are today, absent a global effort to push back against them," CIA Director Mike Pompeo said in October. "It's now a matter of thinking about, how do you stop the final step?"

Foreign policy leaders and observers of all sides agree that Trump can issue any military order unilaterally in the face of an imminent attack by North Korea. But several Democrats argued he should not be allowed to issue a pre-emptive attack of any kind, but particularly involving nuclear weapons, if an attack is not imminent.

“I would like to be able to tell my constituents and the American people we have a system in place that prevents an impulsive and irrational decision to use nuclear weapons,” Maryland Sen. Ben Cardin, the top Democrat on the committee, said Tuesday. “Unfortunately, I cannot make those assurances today.”

American policymakers have agreed, for decades, that North Korea should never be allowed to obtain the ability to strike the U.S. homeland with a nuclear weapon. A North Korean diplomat who defected from the regime in 2016 said dictator Kim Jong Un doesn’t intend to attack the United States, but instead wants to use the weapons to blackmail American leaders into withdrawing U.S. forces from South Korea.

“While Kim Jong Un has already long had the tools to destroy South Korea effectively, he also believes it is necessary to drive American forces out of the peninsula,” Thae Yong Ho told the House Foreign Affairs Committee during a Nov. 1 hearing. “And this can be done, he believes, by being able to credibly threaten the continental United States with nuclear weapons.”

It’s not clear what would happen if Trump were to conclude that a pre-emptive nuclear attack had become necessary and his military subordinates decided it was not warranted under the law. “I always assumed that if issues got raised, at the most senior level, that we would be able to resolve those issues,” Kehler said.