Gen. Mark Milley is being criticized for taking actions to forestall the possibility of an inappropriate nuclear launch order by President Trump. The criticism is based on steps the general allegedly took, as described in Bob Woodward and Robert Costa’s new book, “Peril.” Gen. Milley was ostensibly concerned that Mr. Trump was unstable and might order a nuclear launch for political reasons. The general told Congress last month that because he believed China had unwarranted worries of a U.S. attack, he acted to “de-escalate” the situation and contacted his Chinese counterparts to indicate that no attack was planned.

The critics are missing the point. The overriding issue is not whether Gen. Milley was correct in his assessment, or whether he was authorized to take the reported actions, but what the consequences could have been if his concern had been warranted. It is not hyperbole to say that the consequences could have been a profound tragedy and, in the worst case, the end of civilization.

This is not the first time unauthorized actions might have been taken to stop an unstable president from starting a nuclear war. During the end of Richard Nixon’s presidency, when the president was known to be drinking heavily, Defense Secretary James Schlesinger is reported to have called the commander of the Strategic Air Command and instructed him that if he got a launch order from Nixon, he shouldn’t take any action without first checking with Schlesinger or Secretary of State Henry Kissinger.

The defense secretary had no authority to do so—and couldn’t be sure the general would follow his orders. But he felt he had to do something to head off such a possibility. Schlesinger wasn’t criticized because neither he nor the general made his action public. From my conversations with Schlesinger many years later, I am convinced the story is true.

A related incident took place in the Soviet Union in 1983. Lt. Col. Stanislav Petrov, on duty at the Soviet missile warning station, declined to alert his commanders to an attack assessment on the radar. Petrov believed the assessment was false and feared President Yuri Andropov might launch a response without waiting for further confirmation. This story is recounted in the documentary “The Man Who Saved the World.”

Petrov was correct, which soon became apparent. One can only speculate what might have happened if the incorrect warning had reached Andropov in the middle of the night. Petrov used his judgment instead of following orders because he understood the possible civilization-ending consequences if his radar or computers were wrong, which they were. Yet he was not commended for “saving the world”; he was disciplined for not following policy.

I personally experienced three different false alarms while undersecretary of defense during the Cold War. In 1980 the watch officer at the North American Air Command woke me at 3 a.m. and said his computers indicated hundreds of ICBMs were on the way from the Soviet Union to the U.S. He went on to say that he concluded that it was a false alarm. He wanted me to help determine what had gone wrong with his computer, which took several days—in this case, a faulty chip. We replaced the chip and soldiered on, but with profound uneasiness about the reliability of our warning system.

Gen. Milley should be praised, not condemned, for seeking to avoid a nuclear holocaust. America is fortunate to have military leaders with the judgment and courage to take such actions, even when it means disregarding policy. Punishing Gen. Milley could make it harder for his successors to act with the same courage.

The problem is not that Gen. Milley deviated from policy; the problem is the policy. No president should have the sole authority to start a nuclear war. The Constitution gives war-making responsibility to Congress, and launching a nuclear strike is certainly starting a war. The rationale is that a president may have only minutes to decide whether to launch an ICBM if, say, a surprise attack is detected. But we have more than enough submarine missiles to respond with devastating force to any attack, even one that wiped out our silos. We can ride out any presumed attack until we are certain it is happening.

We shouldn’t launch a response merely because our warning systems suggest that an attack has begun. False alarms have happened and will continue to happen, particularly in the age of cyberwars. We should never launch our nuclear missiles in haste, and our force structure is powerful and diverse enough that we don’t have to.

A decision to launch nuclear forces requires serious deliberation and appropriate consultation. We should reject a policy that gives the president the sole authority to launch nuclear forces and instead establish one that allows for deliberation, including consultation with leaders in Congress. Such a policy would pre-empt the perceived need of military leaders to break the rules to save the world.

Mr. Perry served as U.S. defense secretary, 1994-97.