SEOUL, South Korea — Neat certainties are rare in the North Korean nuclear crisis, which for decades has simmered and occasionally boiled over, without resolution.

So it was jarring to see the absolute confidence with which America's top Pacific commander described the ability of a contentious U.S. missile defense system, scheduled to be up and running in days in South Korea, to shoot down North Korean missiles.

"If it flies, it will die," Adm. Harry Harris told U.S. lawmakers at a hearing Wednesday.

Like nearly everything associated with the world's last Cold War standoff, the truth is muddier. The Terminal High Altitude Area Defense system, or THAAD, has its limits and unknowns.

However, Harris does have some data to back up his bold statement.

After an early redesign, THAAD was reportedly successfully tested 12 times, according to Jonathan McDowell, an astrophysicist at the Harvard-Smithsonian Center for Astrophysics.

A controlled test, however, is a much different matter than an actual war, where large numbers of missiles will be fired with little or no warning.

"Things that work well at home on the test range don't always go as smoothly when deployed," McDowell said.

A salvo of multiple North Korean short-range missiles, for instance, could overwhelm THAAD, said David Wright, co-director of the Union of Concerned Scientists' Global Security Program.

THAAD will also be deployed about 125 miles south of Seoul, whose greater metropolitan area, about an hour from the heavily armed border, is home to 25 million people. "It cannot engage missiles fired at Seoul, so it offers no additional protection of the city," Wright said.

Some scientists are even blunter.

Harris' comments about THAAD's capabilities "are technically incorrect," said Theodore Postol, a professor at the Massachusetts Institute of Technology. "The THAAD interceptor is very easily defeated by either causing a missile to tumble end over end or by intentionally fragmenting a rocket into pieces."

THAAD's capabilities as a defense system "can be expected to be very low, probably zero or close to that," Postol said.

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