Corker, after all, is not a passive spectator; he’s the chairman of the Senate Foreign Relations Committee. “The Congress holds the ultimate power for war,” Jerry Taylor, president of the Niskanen Center, a libertarian think tank, told me. “Though they have more or less delegated that power away to the executive branch, they can take it back.”

They could start with a pair of bills introduced by Massachusetts Senator Ed Markey and California Representative Ted Lieu, both Democrats, prohibiting the president from launching a nuclear first strike without a congressional declaration of war. So far, the only Republican to sign on in either chamber is Congressman Walter Jones of North Carolina. But given how little faith Senate Republicans have in Trump’s judgment, they have a duty to take up this legislation or develop an alternative. “Increasingly, senators and members of Congress are going to come to the conclusion that there has to be a firewall that is erected so that a single human being cannot impulsively launch nuclear weapons,” Markey told me.

Despite its overall record of weakness, Congress has already acted on one occasion to curb Trump’s worst foreign policy impulses. In July, Republicans voted overwhelmingly for a bipartisan bill that, among other things, limited Trump’s ability to unilaterally lift sanctions on Russia. Tying Trump’s hands on nuclear weapons would be a far more aggressive step, but it’s one that members of Congress who are mindful of this moment’s profound peril should take.

Of course, “should” is the key word here. There are plenty of things that Republicans should do about Trump, including impeaching him for violating the emoluments clause of the Constitution. We’ve grown so inured to Republican politicians’ persistent refusal to put the welfare of the country above their re-election prospects and lust for tax cuts that complaining about it feels banal and naïve.

But Corker’s expression of alarm is a reminder that we are teetering on the cusp of horror. He made it clear that Trump’s tweeted provocations of North Korea are impulsive rather than strategic. “A lot of people think that there is some kind of ‘good cop, bad cop’ act underway, but that’s just not true,” he said. We need to take seriously the possibility that Trump might cavalierly start a war that could kill millions of people. It would be a human calamity of inconceivable, history-bending scale, and it would leave America as a hated global pariah. Now that Corker has admitted that Trump cannot be trusted with the power he holds, he and other Republicans have no excuse not to try to take that power away.