The Patriot, first used in the Gulf War, is stationed in South Korea, Kuwait and Saudi Arabia and kept on hot alert -- ready to fire at a moment's notice -- to protect U.S. troops from a possible missile attack by North Korea or Iraq.
CBS News National Security Correspondent David Martin says routine testing revealed that half the Patriots on alert had faulty parts.
Lt. Gen. Paul Kern says, Our concerns were for these deployed missiles that were in the hot status, that it was not worth the risk to leave them, that we would replace them.
Raytheon, the maker of the Patriot, guarantees the missile can be kept on hot alert for six months and still work when fired. But the Army had kept its Patriots on hot alert for much longer than six months -- years, in some cases -- and parts were beginning to fail at an alarming rate. Ten days ago, the Army decided it couldn't trust the missiles any longer. Working in secret, it replaced the hundreds of Patriots kept on alert with missiles out of its war reserve stocks.
Kern says, We quietly swapped out those missiles and that was completed (Tuesday).
The Army is not sure how much it will cost to fix the bad missiles, but the parts alone cost $100,000 a missile, and there are several hundred.
Seven other countries also rely on the Patriot. The Army has now notified them of the problem -- but not until after it had first replaced all of its own Patriots.
Asked why Israel, Saudi Arabia, Kuwait, the Netherlands, Germany, Japan and Taiwan were not told about the possible vulnerability of their Patriot missiles, Kern replied: We did what we thought was a very fair assessment of keeping the security of our forces and or allies in check, and we think we did it the right way.
The Patriot is used to defend against aircraft and short-range ballistic missiles at medium and high altitudes. An upgraded version, called the PAC-3, is being developed to provide a greater capability against short-range missiles. Kern said the PAC-3 version is expected to be ready for use next year.