If ballistic missile defense (BMD) systems are developed to the point where they work dependably, they nonetheless will be effective only against limited numbers of incoming enemy missiles, and BMD systems later will be checkmated by weapons not yet imagined.

That was the assessment of Zbigniew Brzezinski, former White House national security adviser and Center for Strategic and International Studies counselor and trustee, speaking before a Defense Daily media breakfast sponsored by BAE Systems.

The drive by each nation to obtain the ultimate weapon yields but temporary success at best, because every new advance in weaponry inevitably is eclipsed by a system more advanced, said Brzezinski, national security adviser to President Carter and informal adviser to presidents and Pentagon leaders since.

He spoke on a day when the Missile Defense Agency announced the successful test of a Lockheed Martin Corp. [LMT]-Raytheon Co. [RTN] BMD system against a target missile in its terminal (descent) stage. (Please see full story in this issue.)

Whenever some nation flatters itself that it has brought forth the ultimate military development, such as BMD, “in five years Weapon Y comes along” and supersedes it, Brzezinski said.

“You don’t have anything that lasts” indefinitely, he added. He didn’t say just what might come along to displace or challenge BMD systems.

Further, even before that time, BMD systems (assuming they can be developed to work dependably) can be overwhelmed, he noted.

Against a few enemy missiles, BMD systems may be effective in destroying the threatening weapons before they reach American targets, Brzezinski said. But “it’s very difficult to do it against huge attacks,” where an enemy would launch “wave after wave” of enemy missiles.

Pentagon strategists are concerned by North Korea saying it has developed nuclear weapons. Further, the isolated regime has fired a missile over Japan, landing in the Pacific Ocean. And it is working to develop a Taepo Dong-2 missile variant that would be able to strike targets in North America.

Here, U.S. anti-missile systems may be effective, Brzezinski said, because weapons fired by North Korea “are not likely to be very numerous. It is not likely to be a large attack.”

But a U.S. missile shield wouldn’t be effective against Russia, for example, which possesses hundreds of intercontinental ballistic missiles, he noted.