The national missile defense (NMD) is a millitary strategy discussed in the
United States since the 1960s. The basic idea is to shield the U.S. against incomming missiles by shooting them down as they approach the country. It is widely disputed whether the NMD concept is feasible [1].
[1] Jack Mendelsohn,
And it still won't work, [Bulletin of the Atomic Scientists]
?, Vol. 55, No. 4 (1999)