Defense Secretary Jim Mattis and Secretary of State Rex Tillerson testify before the Senate Foreign Relations Committee on Capitol Hill on Monday. (Jonathan Ernst/Reuters)

In testimony Monday, top Trump administration officials confirmed that if President Trump decided to strike North Korea, even with a nuclear weapon, there likely would be no way Congress or anyone else would be able to stop him. For at least some in Congress, that’s a matter of urgent concern.

Senators on both sides of the aisle pressed Secretary of State Rex Tillerson and Defense Secretary Jim Mattis about whether they believed Trump has the authority to initiate a preemptive or preventive strike on Pyongyang and what exactly would happen if he made that decision. But they left their Senate Foreign Relations Committee hearing without firm answers. Their level of anxiety reflects a real fear that Trump believes he can attack the North even if there is no imminent threat to the United States.

“There is no authorization for the use of military force against North Korea absent an imminent attack against the United States or against U.S. forces in this region,” Sen. Benjamin L. Cardin (Md.), the ranking Democrat on the committee, said at the beginning of the session.

He asked Tillerson and Mattis if they agreed. Tillerson said yes. Mattis said the president has only “Article 2” authority, referring to Trump’s constitutionally mandated duty to protect the nation. Several senators reminded Tillerson and Mattis such authority is interpreted as authorizing the president to use military force only in response to an attack on American citizens or interests, or in the case an attack is “imminent.”

The definition of “imminent” is crucial because top Trump administration officials constantly say that Kim Jong Un cannot be allowed to possess the capability to strike the United States homeland with an intercontinental ballistic missile topped with a nuclear warhead. That red line seems to suggest that Trump might attack when North Korea acquires that capability, not when the regime of Kim Jong Un is actually set to use it.

“We all understand that ‘imminent’ has some subjectivity,” Sen. Tim Kaine (D-Va.) told me after the hearing. “But you can’t stretch it to mean to prevent them from the ability to attack us sometime in the future.”

Pressed by Sen. Chris Murphy (D-Conn.) on whether the Trump team would see North Korea’s mere possession of such a capability as an “imminent” threat, Tillerson said it would have to be a “fact based” analysis at the time.

“The possession could be sitting in an underground, not ready to be used position. Or possession could be sitting upright … about to be launched,” Tillerson said.

He also said that historically, Article 2 has been used not just to respond to attacks or prevent an imminent attack but also for circumstances that do not rise to the level of a declaration of war.

“And I think that’s the circumstance that we have in the peninsula today in North Korea,” Tillerson said.

The Trump administration relied on Article 2 when attacking the Assad regime in Syria in April, but Kaine told me that, despite months of pleas, the administration has yet to send Congress any detailed legal justification for that strike.

Murphy is pushing new legislation with several other Democrats that would specifically require Trump to obtain congressional authorization before striking North Korea, absent an imminent threat. But it’s not just Democrats who are concerned.

Committee Chairman Bob Corker (R-Tenn.) is clearly worried that Trump’s behavior could lead to armed conflict with North Korea. Last month, he warned Trump’s reckless rhetoric could set the United States “on the path to World War III.” He also said officials such as Tillerson and Mattis “separate our country from chaos.”

At Monday’s hearing, Corker pledged to hold a separate hearing to determine exactly what would happen in the case that Trump decides he wants to go to war with North Korea. Corker said the process of deciding to strike could take as little as 15 to 20 minutes and he wanted Congress’s role to be understood.

Sen. James E. Risch (R-Idaho) also pressed Tillerson and Mattis for details about what would happen if North Korea actually did attack. That sparked a debate over whether the United States would use nuclear weapons first, even if the attack from North Korea was a conventional one.

Sen. Edward J. Markey (D-Mass.) has a bill that would require congressional authorization before the president could use nuclear weapons first in any conflict.

“Since the dawn of the nuclear age seven decades ago, we have been relying upon cooler heads and strategic doctrine to forestall the unthinkable. But too often those kind of ad hoc measures seem less reassuring than ever,” he said.

Mattis said that while there have not been discussions yet about using nuclear weapons first against North Korea, he “could imagine it.” He added that congressional oversight does not equate to operational control.

If Trump decides to attack North Korea, there is ultimately no person or institution that will likely be able to stop him. But using military force just because Kim Jong Un had achieved the capability to strike the U.S. homeland would be legally unjustified — and horribly unwise.