Defense Primer: Strategic Nuclear Forces.

The article reports that the United States has maintained a triad of strategic nuclear delivery vehicles since the early 1960s, consisting of long-range land-based intercontinental ballistic missiles (ICBMs), long-range submarine-launched ballistic missiles (SLBMs), and long-range heavy bombers. The number of nuclear warheads on these vehicles peaked in the late 1980s has been decreasing ever since due to U.S.-Russian arms control agreements and post-Cold War changes in requirements.