From the Document: "The United States has been developing and deploying ballistic missile defenses (BMD) to defend against enemy missiles continuously since the late 1940s. In the late 1960s and early 1970s, the United States deployed a limited nuclear-tipped BMD system to protect a portion of its U.S. land-based nuclear ICBM (intercontinental ballistic missile) force in order to preserve a strategic deterrent against a Soviet nuclear attack on the Homeland. That system became active in 1975 but shut down in 1976 because of concerns over cost and effectiveness. In the FY1975 budget, the Army began funding research into hit-to-kill or kinetic energy interceptors as an alternative--the type of interceptor technology dominates U.S. BMD systems today."
CRS In Focus, IF10541
Congressional Research Service: https://crsreports.congress.gov/