President Bush had barely finished announcing his intention to withdraw the United States from the Anti-Ballistic Missile (ABM) Treaty when the cry went up among missile-defense foes: Why now?

After all, they said, it's not as if we're about to deploy a missile shield today. We have more testing to do. And here's the president, pulling out of an agreement that arms-control proponents say has kept the world safe for decades. He's "rolling the diplomatic dice," The New York Times wrote. What's the rush?

It's a fair question. And the answer boils down to this: He had no choice.

Indeed, our security needs demanded he take this step. Some observers may hail the ABM Treaty as the "cornerstone of strategic security," but the fact is that the treaty no longer addressed the security risks facing America -- or the world -- in the new millennium.

The treaty, let's recall, enshrined the doctrine of "mutual assured destruction" -- the notion that a policy of deliberate vulnerability would reduce the number of missiles in the world. In theory, if we agreed not to deploy a missile shield, our enemies would stop building more missiles in an escalating attempt to overcome it.

But it didn't work out that way. The number of strategic warheads in the Soviet arsenal grew from about 2,000 in 1972 -- the year we signed the ABM Treaty -- to 12,000 in 1990. The United States, meanwhile, was swelling its own arsenal. Far from preventing an arms race, the ABM Treaty appears to have caused one.

And -- in a point critical to understanding why the president had to jettison the treaty sooner rather than later -- this missile buildup hasn't been confined to the United States and the Soviet Union, the only nations to sign the treaty.

China, for example, has had a long-range missile program in place since the mid-1970s, and it has been modernizing and expanding its ICBM force since the early 1990s. It has no fewer than four programs to develop missiles capable of striking U.S. soil.

North Korea's been busy as well. Its missile-development program actually poses the most urgent threat to the United States. Communist government officials in Pyongyang have five active ballistic missile programs in place, including the Taepo-Dong 2, which a congressionally appointed commission has warned could reach cities and military bases in Alaska and Hawaii. A "lightweight variation" of this missile, the commission says, may be able to reach "U.S. territory in an arc extending northwest from Phoenix, Arizona, to Madison, Wisconsin."

Let's not forget North Korea's Nodong missiles -- not only targeted at American troops but sold to Pakistan, Libya and Iran. Or the arms-race that erupted between Pakistan and India in 1998. Or the missile programs in full swing in Syria and Iraq.

The point is, all this has taken place in a world ruled by the ABM Treaty. Yes, the "big one" -- a Cold-War nuclear showdown between the United States and the Soviet Union -- never occurred. But it's clear the treaty's ability to preserve "strategic stability," in a one-superpower world where missile proliferation is common, has been wildly overblown.

In fact, we'll be safer without the treaty. Ask yourself: Will the North Koreas of the world continue to waste their money building missiles they know we'll be able to knock out of the sky?

OK, critics may reply, but these threats have been building for a while. Why withdraw from the ABM Treaty before we have a system ready to go? Let's test some more. But that's just the point. Under the treaty, we can't test some of our most promising missile-defense systems. It allows only one ground-based system -- and even this system had numerous constraints. There's no way to create the layered system the president envisions, with land-, sea- and space-based components, without violating the treaty.

Ironically, Russia seems more amenable to the president's decision than many of his hyperventilating critics. The Union of Concerned Scientists, for example, says our withdrawal "could block even deeper cuts in U.S. and Russian nuclear arsenals." Yet in January both nations will sit down to discuss not if they'll be cutting their arsenals, but how deeply and how fast.

Sounds like President Bush rolled the "diplomatic dice" -- and won.

Jack Spencer is a policy analyst in the Davis Institute for International Studies at The Heritage Foundation, a Washington-based public policy research institute.