Nonproliferation: Curbs on the Big Rockets

It is a nightmare of the modern age: a group of terrorists, say, or a renegade nation obtains a nuclear warhead and then acquires the means to deliver the deadly device. Last week the U.S. and six allies (Japan, Britain, France, West Germany, Italy and Canada) announced an agreement that could make that scenario less likely. Each has promised not to export any missiles powerful enough to carry a 1,100-lb. nuclear warhead more than 190 miles.

The negotiations, conducted in extraordinary secrecy, lasted four years. Though the agreement is a small step forward in stemming nuclear proliferation, it has not been endorsed by several countries that already produce missiles capable of carrying nukes, notably the Soviet Union, China and probably India and Brazil.