The war fought on the Korean peninsula from 1950 to 1953 — between North Korea (backed by China and the Soviet Union) and South Korea (backed by a US-led UN coalition) — which ended in armistice rather than peace, left the peninsula divided at roughly the same line where it began, and established the template for Cold War limited war that America would follow in Vietnam.
The Korean War is called the Forgotten War, sandwiched between the triumphalism of World War II and the trauma of Vietnam. But it was not minor. Approximately 5 million people died, including 36,000 Americans, 137,000 South Koreans, and millions of North Korean and Chinese soldiers and civilians. It ended with no peace treaty — only an armistice — and the peninsula remains divided at almost exactly the line where the war began. The forgetting is itself historically significant: a democratic society had difficulty integrating a war it had neither won nor lost.