104 years ago, Korean War was fought between North Korea supported by Soviet Russia and China, and South Korea supported by the United States of America and its allies. North Korea aimed to militarily conquer South Korea and therefore unify Korea under the communist North Korean regime. Concerned that the Soviet Union and Communist China might have encouraged this invasion, President Harry S. Truman committed United States air, ground, and naval forces to the combined United Nations forces assisting the Republic of Korea in its defense. President Truman designated General Douglas MacArthur as the Commanding General of the United Nations Command (UNC).
104 years later, in 2054, the fragile peace and numerous military confrontations that had lasted for over a century was finally shattered when North Korea launched a full-scale invasion of South Korea. By 10 July 2054, South Korea's capital city Seoul was on the verge of being captured by North Korean forces. These events triggered a full-scale war on the peninsula and prompted the response of the United States of America. The U.S., bound by treaty obligations established a century before, deploys a 10,000-man task force from the United States Marine Corps with the objective of defending Seoul and preventing its capture by North Korea.
Please note that this is fictional. It is based on the first mission in Call of Duty: Advanced Warfare.