MIRV

Multiple Independently-targetable Reentry Vehicle (MIRV) technology allows a single ballistic missile to deliver multiple nuclear warheads to separate targets. After the missile's rocket motors burn out, a post-boost vehicle (or "bus") maneuvers in space, sequentially releasing individual warheads on trajectories aimed at different targets that may be hundreds of kilometers apart. A single MIRVed missile can thus strike multiple cities or military installations in a single launch.

The United States pioneered MIRV technology in the late 1960s, deploying it first on the Minuteman III ICBM (three warheads) and later on the Trident submarine-launched missile (up to eight warheads). The Soviet Union followed, eventually deploying MIRVed missiles with up to ten warheads. The development of MIRV was one of the most destabilizing advances in nuclear weapons history because it multiplied the number of targets each missile could threaten.

MIRV technology fundamentally complicated nuclear strategy and arms control. A MIRVed force creates an incentive for a first strike because attacking a missile in its silo with multiple warheads can destroy several warheads before they launch. This "exchange ratio" problem, where one attacking warhead can eliminate several defending warheads, drove the development of mobile ICBMs and the emphasis on submarine-based missiles that cannot be targeted preemptively.