The United States MX (MX-missile experimental) program has had a long and controversial history. Beginning in the 1960s, U.S. military experts anticipated growing improvement in the accuracy and number of Soviet missile systems, to the point where they would be able to attack and destroy the concrete underground silos within which the land-based U. S. intercontinental ballistic missiles (ICBMs) were housed. It therefore seemed necessary that new missiles should be deployed in such a way that they would be invulnerable to Soviet attack.