38th Parallel North/ divided by Russia, United Kingdom, and then taken by Japan...and then...and then...etc...it's understandable that the Koreans can be a bit paranoid.

KOREA

The 38th parallel was first suggested as a dividing line for Korea in 1896.^[2] The Russian Empire was attempting to pull Korea under its control, whereas the Japanese Empire had just gained recognition of its rights in Korea from the British Empire. In an attempt to prevent any conflict, the Japanese government proposed to the Russian Empire that the two sides split Korea into two disjoint spheres of influence along the 38th parallel. However, no formal agreement was ever reached, and the Japanese Empire took full control of Korea in 1910.

The land on the left side of the boundary in this picture belongs to South Korea while on the right side it belongs to North Korea

After the surrender of Japan in August 1945, the 38th parallel was established as the boundary by Dean Rusk and Charles Bonesteel of the U.S. State Department - War - Navy Coordinating Committee in Washington, D.C. during the night of the 10th of August 1945, four days before the liberation of Korea. This parallel divides the Korean peninsula roughly in the middle. In 1948, this parallel became the boundary between the newly-created countries of North Korea and South Korea. On 25 June 1950, after a series of cross-border raids and gunfire from both the Northern and the Southern sides, the North Korean Army crossed the parallel and invaded South Korea. This sparked a United Nations resolution against the aggression and the Korean War, with United Nations troops (mostly Americans) helping to defend South Korea.^[3]
During World War II, the Korean Liberation Army had been preparing an assault against the Japanese Army that was occupying Korea — in conjunction with U.S. Office of Strategic Services — but the Surrender of the Japanese Empire canceled the execution of this plan. The Korean government's and U.S. government's goals had been achieved with Japanese surrender on August 14, 1945, formalized in Tokyo Bay on September 2, 1945.