In a written statement released Tuesday, the president called the initial deployment a "modest start." But he said He said it will help protect the American people from threats posed by hostile states or terrorist groups that seek weapons of mass destruction.
The plan calls for basing 20 ground-launched missile interceptors in Alaska and California over a two-year period starting in 2004. Up to 20 sea-based interceptors would also be deployed.
Washington has also asked to use radar sites in Britain and Greenland, a Danish territory, to track missiles that could threaten U-S territory. An early warning radar at the Thule Air Base in northern Greenland would have to be upgraded.
A recent test of the anti-missile system failed, but Defense Secretary Donald Rumsfeld says deployment should begin now, as the system is being perfected.
Last week's failure of an anti-missile test over the Pacific Ocean was the third in eight major tests of the system. Critics of the multi-billion dollar system say it is not effective enough to begin deployment, and may never work.
Once in place, the system -- if it works-- would allow the military to shoot down any long-range missiles fired at the United States.
The decision to begin construction on the missile defense system comes one year after the United States announced its withdrawal from the 1972 Anti-Ballistic Missile treaty. The treaty barred construction or testing of long-range missile defense systems by either the United States or Russia.