For generations, the U.S. nuclear arsenal was viewed as the ultimate deterrent. No one would dare attack us, we thought, out of fear of massive retaliation.

But deterrence requires that a potential foe believe we might use powerful weapons. The strength of some nuclear devices makes that unthinkable. The bomb dropped on Hiroshima during World War II had an explosive yield of no more than 18,000 tons of TNT. But both the United States and Russia have weapons with yields of more than 48 million tons of TNT.

A new — and puzzling — controversy has arisen regarding a Pentagon announcement a few days ago. It was revealed that a new low-yield nuclear warhead, the W76-2, is being placed on some missiles carried by U.S. nuclear submarines. The W76-2 can explode with a force roughly half that of the Hiroshima bomb.

Obviously, use of such a weapon would be a horrible thing. But, as the Pentagon pointed out, it provides a real deterrent to our enemies because they know that, should the need arise, it might be used.

Critics say the W76-2 is dangerous simply because it is more likely than larger-yield weapons to be used.

“I maintain that this is one weapon that will not add to our national security, but would only increase the risk of miscalculation with dire consequences,” commented U.S. Sen. Jack Reed, D-R.I.

But the W76-2, as one might guess from its number, is not really a new concept. Its predecessor, the also low-yield W76, was first manufactured in 1978. One wonders why it took critics such as Reed more than 30 years to become aware of tactical nuclear devices.

No one wants a war in which nuclear bombs are detonated. But, like it or not, weapons such as the W76 and the W76-2 are necessary to achieve the deterrence on which much of our national security rests.