Ground-Based Midcourse Defense (GMD) is the United States' sole operational defense against limited intercontinental ballistic missile (ICBM) attacks on the U.S. homeland. Operated by the Missile Defense Agency (MDA) and the U.S. Army's 100th Missile Defense Brigade, it relies on Ground-Based Interceptors (GBIs) deployed in silos at Fort Greely, Alaska, and Vandenberg Space Force Base, California.
The system targets enemy warheads during the midcourse phase of flight — the long, exo-atmospheric coast between booster burnout and atmospheric reentry. A GBI carries an Exoatmospheric Kill Vehicle (EKV) that maneuvers in space to collide directly with the incoming warhead, destroying it through kinetic energy ("hit-to-kill") rather than an explosive warhead. Targeting data is fused from a network of sensors including the Sea-Based X-band Radar, Upgraded Early Warning Radars at Beale, Fylingdales, Thule, Clear, and Cape Cod, the AN/TPY-2 radar, and Space-Based Infrared System (SBIRS) satellites.
GMD became operationally available in 2004 under President George W. Bush, following the U.S. withdrawal from the 1972 Anti-Ballistic Missile (ABM) Treaty in June 2002. As of the early 2020s, 44 GBIs are deployed. The Pentagon is developing the Next Generation Interceptor (NGI) — awarded to Lockheed Martin in 2024 — to replace the aging EKV fleet and expand the deployed force.
GMD is explicitly postured against limited threats from states such as North Korea and potentially Iran, not against the large arsenals of Russia or China. Russian and Chinese officials nonetheless cite GMD as undermining strategic stability and have invoked it to justify development of hypersonic glide vehicles and MIRVed systems.
Test performance has been mixed: per MDA data, intercept tests have succeeded roughly half the time in controlled conditions, and critics — including the American Physical Society and the Union of Concerned Scientists — note vulnerabilities to decoys and countermeasures. Supporters argue it provides meaningful insurance against rogue-state breakout.
Example
In November 2020, the Missile Defense Agency conducted FTM-44, in which an SM-3 Block IIA — not a GMD interceptor — engaged an ICBM-class target; the test informed debate over GMD's role as the dedicated homeland ICBM shield deployed at Fort Greely, Alaska, and Vandenberg.
Frequently asked questions
GMD intercepts long-range ICBMs in midcourse from silos in Alaska and California. THAAD targets shorter-range missiles in terminal phase, and Aegis BMD operates from ships against short- to intermediate-range threats, though the SM-3 Block IIA has demonstrated some ICBM capability.
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