Over the last two decades, the US has continuously invested in technology development and deployment of layered missile defense systems for the protection of its homeland and its forces abroad. It faces threats from countries, such as North Korea, Iran, Russia, and China as these countries are deploying offensive missile systems. The US continues to make significant progress in the development and deployment of active missile defense capabilities. Some of the major missile defense systems deployed by the US include the Terminal High Altitude Area Defense (THAAD), Aegis Sea-based Missile Defense, Aegis Ashore, and Patriot Advanced Capability-3 (PAC-3) systems.