The Pentagon has spent more than $40 billion on an anti-missile defense system designed specifically to thwart a nuclear strike by North Korea or Iran. Here is a look at the system’s origins, how it’s supposed to work and the technical problems that have bedeviled it.

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A flurry of recent missile tests by North Korea has set nerves on edge and stirred fresh concern about whether U.S. defenses could protect Americans against a sneak attack. North Korea has detonated nuclear devices and is trying to develop long-range missiles capable of reaching the United States.

The Pentagon has spent more than $40 billion on the Ground-based Midcourse Defense system — GMD for short. It’s designed specifically to thwart a nuclear strike by North Korea or Iran. Yet there are grave doubts about whether it’s up to the task.

Here is a look at the system’s origins, how it’s supposed to work and the technical problems that have bedeviled it.

Q: What exactly is GMD supposed to do?

A: It’s designed to defend the United States against a “limited” nuclear attack. That means a strike with a handful of missiles, as opposed to a massive assault of the kind that Russia or China could launch. The United States relies on deterrence — the threat of overwhelming retaliation — to prevent Russia or China from ever unleashing missiles against us. In the case of North Korea or Iran, we would rely on GMD to knock incoming warheads out of the sky.

Q: How would GMD do that?

A: By intercepting incoming warheads in space, just as they’re about to begin their re-entry into the atmosphere. That’s the approximate “midcourse” point in a warhead’s journey from launchpad to target.

Q: What’s an interceptor, and how does it work?

A: The GMD interceptors are 60-foot-tall, three-stage rockets. Each has a 5-foot, 150-pound “kill vehicle” at its tip. In the event of an attack, interceptors would be launched from their underground silos. Once in space, the kill vehicles would separate from their boost rockets and fly independently toward their targets, at speeds up to 4 miles per second.

Q: How many interceptors are there, and where are they based?

A: There are 37 operational interceptors — four at Vandenberg Air Force Base in Santa Barbara County, Calif., and 33 at Fort Greely, Alaska.

Q: How would the kill vehicle destroy an incoming warhead? By blowing it up?

A: The kill vehicles carry no explosives. They’re designed to destroy enemy warheads with kinetic energy, or energy of motion — in other words, by crashing into them.

Q: How would the kill vehicle find the incoming warhead?

A: Satellites and powerful radars, stationed on land and at sea, track airborne objects. The GMD system receives data from these and other sources and would use it to guide the interceptors. The kill vehicle also has an onboard navigation system to help it zero in on its target.

Q: Why are there doubts about GMD’s reliability?

A: Intercepting a warhead traveling at hypersonic speed is a supreme technical challenge. It’s been compared to hitting one speeding bullet with another. GMD has not shown that it could do that dependably.

The system has performed poorly in flight tests, and technical problems keep cropping up. In nine simulated attacks since GMD was deployed in 2004, interceptors have failed to take out their targets six times. And the flight tests are much less challenging than an actual attack would be. They’re carefully scripted for success: The operating personnel know ahead of time when mock warheads will be launched, as well as their size, speed and approximate trajectory.

Defense planners assume that four or five GMD interceptors would have to be launched for every incoming warhead to have a good chance of destroying them all, according to current and former government officials.

Q: What caused the failures in the GMD test flights?

A: It’s no single problem. In some cases, divert thrusters were blamed. These are small rocket motors, four of which are attached to each kill vehicle. They’re supposed to fire rapidly to make course corrections and keep the kill vehicle on course once it’s in space and flying on its own. In some tests, the thrusters’ “rough combustion” of fuel was found to have thrown off the kill vehicles’ onboard guidance system.

A lot of time and money was spent redesigning the thrusters to cure this problem. In January 2016, a flight test was initiated from Vandenberg Air Force Base to check out the new thrusters. The Missile Defense Agency and its lead contractors pronounced the test a success. But as the Los Angeles Times/Tribune Washington Bureau later reported, it wasn’t. Partway through the exercise, one of the new thrusters stopped working, and the kill vehicle veered far off-course.

Bottom line: The kill vehicles are tremendously complex machines. Each one has more than 1,000 components. And because of the speed with which they’ve been produced and deployed, no two are identical.

Q: Why the rush?

A: American scientists had been working on missile-defense technology for decades, with the aim of creating a reliable shield for the U.S. homeland. President Bill Clinton, whose administration supported and funded such research, concluded that the technology wasn’t ready for prime time. But his successor, President George W. Bush, had campaigned on a promise to deploy a homeland missile-defense system quickly, asserting that the country was in imminent danger of a sneak attack by a “rogue state” such as North Korea or Iran.

In late 2002, Bush ordered the Pentagon to field a “set of missile defense capabilities” by the end of 2004. To speed things along, Defense Secretary Donald Rumsfeld exempted the program from the Pentagon’s normal procurement and testing standards.

Q: Who built the interceptors and other components of GMD?

A: A handful of defense contractors have produced the system’s major elements. Raytheon has built the kill vehicles and radars. Orbital ATK has made the boost rockets. Northrop Grumman has provided the worldwide communications links. Boeing has managed GMD as the government’s prime contractor.