Any massive invasion from the North could be stymied by releasing water from South Korean DAMS along the Han River, flooding areas the invaders have to cross.

任何自北边来的大规模进攻都将能被韩国汉江沿线水坝释放的洪水冲溃，并淹没侵入所需要途经的地区。

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In the past, Pyongyang has strongly condemned U.S.-South Korean drills, claiming they are preparations for an invasion of the North.

VOA: standard.2010.05.26

North Korea has threatened revenge for the sanctions and for ongoing routine U.S.-South Korean military drills it considers invasion preparation.

WSJ: South Korea Banks, Media Report Possible Cyberattack

U.S. nuclear submarines ply the region's waters, however, and Washington makes clear that its so-called nuclear "umbrella" over South Korea is meant to deter an attack on its ally by North Korea, whose 1950 invasion ignited the three-year Korean War.

NPR: NKorea: Cold War History With US Justifies Nukes

Like her predecessors, Ms. Park has made the U.S. her first overseas destination after taking office to bolster an alliance that goes back to the 1950-53 Korean War, when the countries fought together against invasion from North Korea.

WSJ: South Korea's Park Travels to U.S. to Bolster Alliance