Decades ago, in the scariest days of the nuclear arms race with Russia, American schoolchildren learned to “duck and cover” under their desks in case an atomic bomb was dropped nearby. Since the end of the Cold War, kids have grown up free of the fear of nuclear attack. But those days may be coming to an end.

New threats have emerged. The first is North Korea, which is believed to have as many as 10 nuclear warheads and recently carried out its third nuclear test. The Pyongyang regime, according to House Intelligence Committee Chairman Mike Rogers, R-Mich., has missiles “that can reach U.S. shores.”

This month, North Korea threatened to launch a “pre-emptive nuclear attack against the headquarters of the aggressor,” meaning the United States.

It’s no comfort that Pyongyang’s arsenal is now in the hands of a young and inexperienced ruler, Kim Jong Un, whose inclinations are a mystery.

Then there is Iran, which is believed to be proceeding toward building its own nuclear stockpile. Its missiles are capable of hitting targets in Israel and Europe, and in 2010 the Pentagon warned that, “with sufficient foreign assistance, Iran could probably develop and test an intercontinental ballistic missile capable of reaching the United States by 2015.”

So it’s no surprise that American military planners are feeling some urgency about addressing the looming specter of nuclear attack. True, the U.S. retains a formidable nuclear capability that would guarantee the instant and total destruction of any government so rash as to launch a missile in our direction. But that doesn’t make it any more comfortable to ponder a mushroom cloud over an American city.

Missile defense is an attempt to buttress the power to retaliate with the ability to fend off incoming warheads before they arrive. Last week the Defense Department said it would spend $1 billion to deploy more missile interceptors along the West Coast to shoot down a North Korean missile, increasing the total number from 30 to 44 in the next four years.

It’s a reasonable and useful step, at a cost that would seem trivial if the system ever were called on to deflect an attack. The defensive system would doubtless give pause to the North Koreans, who would have to contemplate wasting one of their few warheads while assuring a devastating U.S. strike in return. Though the program has been modestly successful in testing, hitting only about half its targets, it is advanced enough to add a significant challenge for Pyongyang.

With regard to Iran, the administration took a different step, scrapping the last phase of a missile defense system that has elicited vigorous objections from the government of Russia — which regarded the program as a threat to neutralize its nuclear weapons. The Pentagon insisted the U.S. decision was based on technical problems, which may be true. But it also may serve to pave the way to better relations and even arms reductions with Moscow.

The danger still exists, of course, but President Barack Obama has made it clear he will take military action if necessary to keep Iran from getting the bomb. If he succeeds in deterring Tehran from that course — or in forcibly preventing it — the European missile shield will not be needed quite so soon.

American missile defense still has a lot of hurdles to surmount before it can offer a reliable safeguard against attack. But even an imperfect system is better than nothing. And no one can doubt the need to keep pursuing it.