Since the 1990s, Terminal High Altitude Area Defense (THAAD) has played a key role in protecting the United States and allies from ballistic missiles.

Formerly known as Theater High Altitude Area Defense, THAAD is an anti-ballistic missile defense system designed to intercept short-, medium- and intermediate-range ballistic missiles.

The system sits in the middle tier of the United States' multilayered defense system: it offers a larger area of defense than does the Patriot missile defense system and supports the exoatmospheric functions of Aegis Ballistic Missile Defense and Ground Based Midcourse Defense System (GMD).

The THAAD system consists of interceptors, launchers, the AN/TPY-2 ground-based X-band radar, a fire control system and support equipment.

THAAD missiles use kinetic energy to destroy incoming missiles rather than relying on a warhead -- a safer option when intercepting nuclear missiles -- and can hit targets up to 200km away.

Although the concept was first proposed in 1987, the US Army awarded defense contractor Lockheed Martin the primary contract for THAAD development in 1992 as a part of an effort to develop a deployable theater missile defense system.

While the system struggled in early testing, it has since had a consistent reliability track record in flight tests.

The Army launched the first THAAD prototype in April 1995, and in June and August 1999, conducted two successful flights in which the system successfully intercepted Hera target missiles, including a unitary target and a separating target.

The Army has continued the development of THAAD.

In recent years, the Army has integrated THAAD into the Link 16 network, a tactical military data network employed by the US, NATO and other allies to communicate relevant air and missile threat data to defense systems.

It provides a secure, jam-resistant, high-speed digital data link that operates at radio and microwave frequencies.

The network enables a range of platforms, including aircraft, surface ships, ground vehicles, missile defence systems, networked weapons and command-and-control networks, to exchange text, imagery and digital voice messages.

The THAAD interceptor's indium-antimonide imaging infrared sensor seeker is also getting upgrades.

In August 2022, Lockheed Martin awarded aerospace engineering firm BAE Systems a contract to develop next-generation infrared seeker technology for THAAD.

The current infrared seeker head -- also made by BAE Systems -- uses advanced electro-optical sensors to seek and lock onto incoming enemy ballistic missiles that are moving as fast as 17,000 miles (27,359km) per hour, working alongside the radar to route the THAAD interceptor to threats, according to BAE Systems.

"THAAD is a highly effective system for mitigating high-speed missile threats, and we’re making it more capable," BAE Systems Precision Guidance and Sensing Solutions Director Greg Procopio said in a statement at the time.

Allied defense

THAAD is a crucial component of the missile defense systems of several major US allies, and has been deployed to Romania, South Korea and Israel.

In 2017, South Korea incorporated THAAD as its second-tier missile defense system, complementing its Patriot PAC-3 systems, to intercept missiles potentially coming from North Korea.

The deployment of THAAD to an ad hoc installation at a golf course drew the ire of China, which retaliated with economic measures.

South Korean announced in August that it would normalize the THAAD site, transforming it into a military base that has operational, support and living facilities for US soldiers operating the system.