It's true that we can't do much to stop North Korea's effort to develop nuclear weapons; we've already failed, in fact. But surely we can thwart the effectiveness of Pyongyang's ballistic missile program. How so? If North Korea builds up its missile offense, we can respond by building up our missile defense.Right here, right now, we should be asking ourselves: Why don ' t we have an effective missile-defense program? Not just Patriot missile batteries and Aegis missile ships, which may or may not work when deployed in dribs and drabs—but a genuinely comprehensive, multi-layered defense system, well funded and constantly tested, that really would do the job, backed by a robust international consensus in favor of self-defense. (As an aside, robust missile defense would be the best argument against nuclear proliferation—the incentive to proliferate is much less if nuclear wannabes know that they couldn't use their new weapons.)The Obama wing of the Democratic Party has always opposed missile defense, of course, going back to the Cold War days. Back then, the left supported the nuclear freeze and deep disarmament, even total disarmament. And if such disarmament efforts didn't succeed, figured liberals of that era, the default scenario of MAD—Mutual Assured Destruction—made for an acceptable "balance of terror." But while such policies might have worked out during the Cold War, against a cautious and conservative Soviet Union, it's harder to argue that arms control and deterrence will keep the peace today, in the face of a multiplicity of volatile actors—not just North Korea and Iran, but also Pakistan and any particularly ingenious terrorist group. So the answer is obvious: missile defense. Now. Why are we afraid of North Korea? Because it might soon be able to fire a missile that nukes Seoul or Tokyo—or maybe San Francisco.And half a world away, Israelis view a nuclear Iran as an "existential" threat to the Jewish state's survival. Maybe the Israelis will attack Iran, and maybe they won't. But even if they do, it's a safe bet that Iran will be a nuclear power within a few years. How so? Because most likely, the Iranians will get a nuke from North Korea, or Pakistan—or from some source we haven't yet identified. Sadly, nuclear technology is not that difficult to develop or obtain. That's why Mohamed Elbaradei, the outgoing director general of the International Atomic Energy Agency, predicted recently that the number of nuclear states could double in the next few years. A doubling would take the "nuclear club" from nine to 18 members. That's a lot of countries to keep cool and calm at all times.So what is the U.S. doing in response to these ominous nuclear trends? For his part, Obama, keeping faith with his ideological roots, called for the complete elimination of nuclear weapons in his April 5 speech in Prague. But in case that complete and verifiable disarmament doesn't break out worldwide, America should do a better job of defending itself. Currently the U.S. spends about $9 billion on missile defense, although the Obama administration plans on pruning back some component programs. (And yes, the danger of quiet shipment of nuclear materials must be addressed, too—here, the Bush administration's Proliferation Security Initiative offers the Obama administration a good platform upon which to build.) But at a time when Israel is routinely pelted with cross-border rockets from low-tech Hamas—and jeopardized by a potential typhoon of rockets from Hezbollah—this is not the time for any nation to be cutting back on missile defense. Instead, this is the time to push forward not only on missile defense, but to push toward what might be called comprehensive defense, protecting against any sort of hostile projectile, large or small, that might cross a frontier. Such technology would make the whole world a safer place. Israel would join us in such an ambitious effort, and so would Japan. That's two of the most technologically adept countries in the world, potentially working in league with us on a Manhattan Project-like endeavor.