An intercontinental ballistic missile, or ICBM, is a type of extremely long-range suborbital missile, designed as a delivery method for nuclear warheads. Within 20 years of the invention of nuclear weapons, ICBMs became the primary method of delivery for the military of the United States and the Soviet Union. Most ICBMs are multiple independently targetable reentry vehicles (MIRVs), which allow one missile to carry multiple warheads, each of which can be designated to a separate target.