More than 13,000 American and South Korean military personnel are taking part in the Key Resolve drill.
South Korea and U.S. Hold Military Drill

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South Korean Army soldiers patrol along a barbed-wire fence near the border village of Panmunjom in Paju, South Korea, March 11, 2013.

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South Korean Army soldiers work on their K-9 self-propelled artillery vehicles during an exercise near the border village of Panmunjom in Paju, South Korea, March 11, 2013.

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South Korean Army soldiers patrol along a barbed-wire fence near the border village of Panmunjom in Paju, South Korea, March 11, 2013.

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South Korean protesters shout slogans during a rally denouncing the annual joint military exercises between South Korea and the United States, near the U.S. Embassy in Seoul, South Korea, March 11, 2013.