SINCE THE congressional supercommittee is reportedly at an impasse, let’s hope its members have used some of their idle time to catch up with the testimony of the nation’s military chiefs at a House Armed Services Committee hearing on Thursday. The chiefs were asked to assess what would be the consequences if $600 billion in across-the-board cuts were imposed on the defense budget — a sequestration currently required by law in the event the supercommittee fails to agree on a debt reduction plan or Congress fails to pass it.

Their answers were blunt: “Cuts of this magnitude would be catastrophic to the military,” testified Army Chief of Staff Gen. Raymond T. Odierno, a former Iraq commander. “My assessment is that the nation would incur an unacceptable level of strategic and operational risk.”

“A severe and irreversible impact on the Navy’s future,” said Adm. Jonathan W. Greenert, chief of naval operations.

“A Marine Corps below the end strength that’s necessary to support even one major contingency,” said Marine Commandant James Amos.

“Even the most thoroughly deliberated strategy may not be able to overcome dire consequences,” said Air Force Chief of Staff Norton Schwartz.

True, the Pentagon brass are known for pushing hard for their funding. But they rarely speak in such apocalyptic tones — and there is good reason to take their warnings seriously. Under President Obama’s budget plan, $465 billion is already due to be cut from military spending over the next decade, from an annual budget now of about $700 billion. That will already require a downsizing of the Army and Marines, the reduction or cancellation of more weapons systems and a shrinking of the Navy to its lowest size in decades. Defense Secretary Leon Panetta, a lifelong budget hawk, is rightly concerned that such cuts may go too far.

If the additional sequestration goes forward, the total reduction could come to $1 trillion. This, Gen. Odierno said, would “almost eliminate our modernizations programs” in the Army, including new armored vehicles. Adm. Greenert said it could force the two U.S. companies that build Navy ships out of business. The Air Force would have to retire some 1,000 aircraft. In all, about 1 million military and civilian jobs would be lost.

Some in Washington may believe the threatened cuts are a paper tiger, since they would not go into effect until 2013 and might be reversed before then. But it’s not that simple: As Adm. Greenert explained, layoffs of personnel and suspensions of programs would have to begin in 2012 to reach the necessary spending level by the start of 2013.

In the meantime, a bad and even dangerous message would be sent to U.S. allies and adversaries. “We’ll have those who attempt to exploit our vulnerabilities,” Gen. Odierno said. “We might lose our credibility in terms of our ability to deter.”

Congress set this bomb in place when it agreed in the summer that half of $1.2 trillion in automatic cuts would be assessed to defense if a debt reduction plan failed to pass this year. Now it has heard from senior commanders just how much damage its explosion would cause. It would be an unconscionable act of political irresponsibility to allow their predictions to come true.