For nearly forty years, one air-defense system has protected the airspace above U.S. forces. Continuously upgraded since introduction, today the Patriot missile system protects against the full spectrum of flying threats, from ballistic missiles to consumer-grade quadcopter drones.

The design of Patriot goes back as far as 1965, when then secretary of defense Robert McNamara authorized development of a new air-defense missile, SAM-D. SAM-D was set to replace the Nike Hercules surface-to-air missile systems and protect against medium- to high-altitude threats. Patriot was intended to be a new-generation system that would incorporate new technologies, including computer control, multiple target engagement, and the ability to operate in an electronic-countermeasures-heavy environment.

As ambitious as SAM-D was, it had a lengthy development cycle. In 1973 it was nearly terminated by Secretary of Defense James Schlesinger, who, among other things, believed that a long-range surface-to-air missile was an Air Force responsibility and that the system was overly complicated. By 1975, it had proved its ability to shoot down targets, and around then was renamed MIM-104 Patriot. Patriot stands for Phased Array Tracking Intercept of Target, although it’s unclear what came first—the name or the acronym. Patriot entered low-rate production in 1980, with the missile first fielded in 1985.

(This first appeared last year.)

Unlike the older Nike Hercules system, Patriot was truck-mounted and mobile. A firing battery could roll into position and be ready to fire in a less than an hour. A battery consisted of a phased array radar set, engagement control station, electric power plant, an antenna mast group, communications relay group, and up to eight launching stations controlling four missiles each.

Recommended: The Story of the F-52 Fighter.

Recommended: The 5 Biggest Nuclear Bomb Tests (From All 6 Nuclear Powers).

Recommended: How Israel Takes U.S. Weapons and Makes Them Better.

Like all medium- to long-range SAMs, Patriot is a radar-guided missile and relies on radar to provide airspace surveillance, target detection, classification and tracking, and finally missile guidance to the actual missile. The AN/MPQ-53 radar system is used for PAC-2, the antiaircraft version of Patriot, while the AN/MPQ-65 radars is used by the ballistic-missile defense version, PAC-3. Both radars use passive electronically scanned array radars that use flat, billboard-like radar arrays similar to those on an Aegis cruiser or destroyer.

The first series of Patriot missiles was approximately seven feet long and had a speed that exceeded Mach 3, and had a range of forty-three miles. The two-thousand-pound missile had a forty-three-pound blast-fragmentation high-explosive warhead designed to destroy enemy aircraft and missiles by detonating in their proximity. This was quickly superseded by an improved version, PAC-1, which entered service in 1988. An even newer version, PAC-2, was introduced in 1990, just in time for Patriot’s baptism by fire.

In August 1990, Iraqi president Saddam Hussein invaded and annexed neighboring Kuwait. This triggered an enormous response by the West, including two Army corps sent to defend Saudi Arabia and later liberate Kuwait. Patriot missiles covered marshalling coalition forces, based in Saudi Arabia. After the air campaign began in January 1991, the Iraqi military began launching Scud short-range ballistic missiles against Saudi and later Israeli targets.

Patriot batteries engaged incoming Scud missiles, and at first it was believed that the missiles were hitting their targets. President George H. W. Bush exclaimed that the Patriot had successfully shot down forty-one out of forty-two missiles. This was later scaled back to 80 percent of missiles aimed at Saudi Arabia and 50 percent aimed at Israel. Later the claims were scaled back even further, to 70 and 40 percent, respectively. Eventually it was conceded that very few of the Patriots destroyed their targets.

What happened? Patriot missiles intercepted through proximity detonation, using their warhead to knock a target out of the sky. The best explanation was that the locally produced Scuds, also known as Al Hussein, suffered from a design flaw that caused them to disintegrate on reentry. This created a larger target cloud of debris and warhead that the Patriot’s proximity warhead could not successfully engage. Critics also charged the missile’s proximity fuse was designed to engage aircraft, but could not engage ballistic-missile warheads moving at high mach speeds.

In light of PAC-2’s shortcomings, a new version was designed to exclusively engage ballistic missile warheads. PAC-3 incorporates “hit-to-kill” technology, destroying ballistic-missile warheads though direct impact rather than proximity explosion. PAC-3 is much smaller than previous versions, with a Patriot launcher able to keep sixteen PAC-3s at the ready where it can only keep four PAC-2s. With a smaller missile comes a shorter range, however, and PAC-3 has a range of only twelve miles.

Today Patriot is on the front lines across the world, from South Korea and Taiwan to Turkey, Israel and Yemen. Israeli Patriot batteries have shot down three drones operated by Hamas and Syria. In addition, Saudi Patriot units have shot down missiles aimed at the country by Houthi rebels. Wary of North Korean ballistic missiles, Japan has deployed Patriot missiles to protect key locations, from downtown Tokyo to the distant Ryukyu Islands.

Last week, Gen. David Perkins, commander of the US Army Training and Doctrine Command, revealed that a U.S. ally had used a Patriot missile to shoot down a commercial quality quadcopter drone. While shooting down a $200 drone with a $3.4 million missile was widely derided as a waste of money, it is admittedly pretty impressive that the Patriot’s radar was able to pick up and direct the missile to successfully destroy such a small object.

Although a replacement for Patriot, when the Medium Extended Air Defense System (MEADS) was developed the U.S. Army opted out of the program, leaving Germany as the only enthusiastic customer for the new air-defense system. It seems likely, however, that MEADS technology will eventually be rolled into future versions of Patriot, especially as threats from ballistic missiles, cruise missiles, aircraft and now even drones continue to evolve. The missile first greenlighted by Robert Strange McNamara will easily serve for another decade.

Kyle Mizokami is a defense and national-security writer based in San Francisco who has appeared in the Diplomat, Foreign Policy, War is Boring and the Daily Beast. In 2009, he cofounded the defense and security blog Japan Security Watch. You can follow him on Twitter: @KyleMizokami.