WASHINGTON—The U.S. is preparing to deploy an advanced missile-defense system to defend American military bases in the Pacific, a defensive signal indicating the Pentagon doesn't believe the threat of a North Korean attack is likely to fade quickly.

The U.S. will deploy the system, known as a Terminal High-Altitude Area Defense battery, or THAAD, to Guam to protect against medium-range missiles from North Korea. It will be the system's first deployment.

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