ALL good things come to an end, and that may one day include America’s military pre-eminence. Although the United States is still by far the world’s strongest martial power, others are catching up. America’s ability to project overwhelming force around the world, which it has taken for granted since the end of the cold war, is now threatened.

In the past America has harnessed technology to offset its rivals’ advantages. Faced with much larger Soviet conventional forces in Europe, it first relied on the superiority of its nuclear arsenal for deterrence (in the 1950s) and then, when the Soviet Union caught up, invested in “deep strike” systems that could spot distant targets and destroy them with precision-guided conventional warheads (from the late 1970s). The Gulf war in 1991 demonstrated the devastating effectiveness of…