The Minuteman intercontinental ballistic missile program has served as a component of US nuclear strategies as land-based ICBMs deployed in missile silos. The Minuteman entered service in 1962 as a deterrence weapon that could hit Soviet cities but the US Air Force modified the missile, boosting its accuracy enough to attack military targets, including Soviet missile silos. The Minuteman-II entered service in 1965 with a host of upgrades to improve its accuracy and survivability in the face of an anti-ballistic missile (ABM) system the Soviets were known to be developing. In 1970, the Minuteman-III became the first deployed ICBM with multiple independently targetable reentry vehicles (MIRV). By the 1970s, 1,000 Minutemen were deployed as part if the US nuclear arsenal. As of 2020 the force has shrunk to under 400 Minuteman-III missiles as they will be replaced by the new Northrop Grumman Ground Based Strategic Deterrent (GBSD) ICBM from 2027 onwards.