(Bloomberg Opinion) -- This month, an intercontinental ballistic missile was fired in the general direction of the Hawaiian islands. During its descent a few minutes later, still outside the earth’s atmosphere, it was struck by another missile that destroyed it.

With that detonation, the world’s tenuous nuclear balance suddenly threatened to come out of kilter. The danger of atom bombs being used again was already increasing. Now it’s grown once more.

The ICBM flying over the Pacific was an American dummy designed to test a new kind of interceptor technology. As it flew, satellites spotted it and alerted an Air Force base in Colorado, which in turn communicated with a Navy destroyer positioned northeast of Hawaii. This ship, the USS John Finn, fired its own missile which, in the jargon, hit and killed the incoming one.

At first glimpse, this sort of technological wizardry would seem to be a cause for not only awe but also joy, for it promises to protect the U.S. from missile attacks by North Korea, for example. But in the weird logic of nuclear strategy, a breakthrough intended to make us safer could end up making us less safe.

That’s because the new interception technology cuts the link between offense and defense that underlies all calculations about nuclear scenarios. Since the Cold War, stability — and thus peace — has been preserved through the macabre reality of mutual assured destruction, or MAD. No nation will launch a first strike if it expects immediate retaliation in kind. A different way of describing MAD is mutual vulnerability.

If one player in this game-theory scenario suddenly gets a shield (these American systems are in fact called Aegis), this mutual vulnerability is gone. Adversaries, in this case mainly Russia but increasingly China too, must assume that their own deterrent is no longer effective because they may not be able to successfully strike back.

For this reason defensive escalation has become almost as controversial as the offensive kind. Russia has been railing against land-based American interceptor systems in places like eastern Europe and Alaska. But this month’s test was the first in which a ship did the intercepting. This twist means that before long the U.S. or another nation could protect itself from all sides.