Last Thursday, the Obama administration announced its long-awaited decision on a European missile defense system against potential Iranian ballistic missiles. In short, it will shelve the Bush administration's plan for a defense against intercontinental-range missiles, and instead, it will field a system designed to intercept shorter-range missiles, on which Iran is making quicker progress.

The White House made the right decision in cancelling the Bush plan, which had a raft of technical and political problems, and it deserves credit for changing course. But the new plan also has significant problems, and the announcement demonstrates that U.S. missile defense policy continues to be based on domestic politics rather than technical reality.

There certainly was no technical...