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South Korean marines aim their machine guns after they landed on the beach during a landing exercise in Taean, South Korea, Monday, June 29, 2015. South Korean navy and marine held a joint landing drill on the west coast on Monday improving their military readiness to protect the Korean Peninsula

Ahn Young-joon/AP

South Korean marines aim their machine guns after they landed on the beach during a landing exercise in Taean, South Korea on Monday. South Korean navy and marine held a joint landing drill on the west coast on Monday improving their military readiness to protect the Korean Peninsula

Members of the South Korean special forces take part in a military drill in Yeongcheon, southeast of Seoul in this May 2, 2013 picture provided by Yonhap. The two Koreas remain technically at war under a mere truce that ended hostilities in their 1950-53 conflict and North Korea, angry at U.N. sanctions and joint South Korean-U.S. military drills, has threatened both countries with nuclear attack in recent weeks.

Han Jong-chan/Yonhap/Reuters

Members of the South Korean special forces take part in a military drill in Yeongcheon, southeast of Seoul. The two Koreas technically remain at war under a truce that ended hostilities in their 1950-1953 conflict. North Korea is angry at UN sanctions and joint South Korean-US military drills, and has recently threatened both countries with nuclear attack.