THAAD, or Terminal High Altitude Area Defense, is designed to intercept ballistic missiles at high altitudes. Patriot systems operate at lower levels and can be used against ballistic and cruise missiles, aircraft and other aerial threats. The Center for Strategic and International Studies recently estimated that fewer than 1,000 Patriot interceptors and approximately 250 THAAD interceptors remained in American inventories. Reuters reported separate figures indicating that roughly 65% of Patriot interceptors had been expended, while the THAAD inventory was at least 38% smaller than it had been at the beginning of the war. Planned industrial expansion would sharply increase the annual output of Patriot and THAAD interceptors, but rebuilding the depleted inventories is expected to take years rather than months.