Why did the US invade the Korean peninsula?
The US invaded Korea twice. The first time was in 1945 when the US and the USSR agreed to invade the Korean Peninsula to defeat the ruling Japanese. The USSR invaded from the north and, as agreed, stopped at the 38th parallel. The US invaded from the south and stopped at the same line. The second time, in 1950, the US only invaded North Korea. The Republic of Korea (South Korea) requested US assistance, so when the US entered South Korea it was by invitation, not invasion. The invasion of North Korea was part of the objective of eliminating the North Korean ability to effectively re-invade South Korea.
