China’s reported test of a nuclear-capable hypersonic weapon has raised alarm about the vulnerability of the U.S. to missile attacks by nuclear-armed adversaries.

The U.S. should be vigilant about emerging threats, but pouring more money into unproven missile-defense technologies isn’t the answer. America’s aim should be to keep its missile-defense capabilities aligned with both fiscal reality and the country’s strategic interests.

Since the early 2000s, America’s missile-defense program has been geared to defend both the U.S. homeland and some overseas military assets from limited missile strikes. That has meant building defenses strong enough to stop an attack of the kind North Korea might contemplate, but not so large that they’d be both unaffordable and destabilizing.