“I can tell you that the United States is fully capable of defending against any North Korean ballistic-missile attack.”

That was White House spokesman Jay Carney reacting to the third nuclear test this year by the self-styled Democratic People’s Republic of Korea, as well as a brash threat from Pyongyang to, well, nuke the United States.

Mr. Carney’s words are reassuring, but the real question is why they were necessary in the first place.

There are two basic problems. One is the administration’s policy on nuclear weapons. The other is the laggardly pace of our missile-defense system.

Begin with the policy, which President Obama outlined in a speech in Prague in 2009: “Let me describe to you the trajectory we need to be on. First, the United States will take concrete steps toward a world without nuclear weapons. To put an end to Cold War thinking, we will reduce the role of nuclear weapons in our national security strategy and urge others to do the same.

“Make no mistake: As long as these weapons exist, the United States will maintain a safe, secure and effective arsenal to deter any adversary, and guarantee that defense to our allies — including the Czech Republic. But we will begin the work of reducing our arsenal.”

The goal of a world free of nuclear weapons is worthy, of course. No one faults the president for that. The problem lies with how this goal will be achieved. Ironically, the president’s approach — which relies on pre-emptive disarmament and places an undue faith in the power of arms-control treaties — ensures a world in which such weapons become more common, not less.

If the United States must maintain a “safe, secure and effective” nuclear weapons arsenal to ensure the security of the nation and its allies, why did the president agree to a New START with Russia that calls for the U.S. to cut its number of nuclear weapons to 1,550 by 2018? Why stand behind the Comprehensive Test Ban Treaty, a pact that other nations flout with impunity while we honor it, and which allows our arsenal to atrophy through disuse?

Our nuclear “modernization” efforts are anything but. Under current policies, replacement systems won’t enter our arsenal until 2030. By then, the United States will have 60-year-old intercontinental-range ballistic missiles, 40-year-old submarine-launched ballistic missiles, and 35- to 70-year-old bombers.

It hardly sounds like a recipe to “deter any adversary.”

China, for example, has announced new defense and internal security budgets. The world’s largest communist country has been posting double-digit annual increases in its military budgets for the past 20 years, and next year is no exception: It will rise by 10.7 percent.

That’s huge. As Heritage Foundation analyst Dean Cheng points out, these consistent increases “have allowed Chinese military modernization to gain by leaps and bounds since 1989. Its modernization ranges from the new aircraft carrier Liaoning to anti-ship ballistic missiles to mechanized airborne combat vehicles.”

North Korea’s efforts to obtain a nuclear arsenal are well-known. Iran, too, continues to try to join the nuclear club, threatening to destabilize the Middle East — and the world beyond — still further.

Are we defenseless against missiles fired at U.S. territory or allies? No. But our missile-defense system isn’t nearly as comprehensive (and, therefore, discouraging to our enemies) as it needs to be.

We need a shield that will intercept missiles during all three stages of flight. It is especially important to pursue a space-based component, which would enable us to shoot down missiles in the ascent phase, when they are moving more slowly. Yet the administration has undercut all missile defense since taking office.

The whole point of such a shield is to put a robust deterrent in place. An effective system discourages threats by convincing potential enemies that there is no point investing in trying to overwhelm our defenses.

The minimal approach pursued by the administration does just the opposite: It encourages them to try harder. In an increasingly dangerous world, why take that chance?

-Ed Feulner is president of the Heritage Foundation (heritage.org).