This article is written to remind the importance of the ROK and US alliance in Korean national security. After World War II, ROK could not defend itself from North Korean invasions and went through a brutal civil war. The ROK and US alliance was established by signing the Republic of Korea-United States mutual defense treaty, and it is the only security alliance for Korea. This article presents three lessons of the Korean War. First, ROK must have a power to defend itself, second, firm cooperative relationships with friendly nations are required, and third, correct analysis of the situation and national concentration are needed. The lessons show proper attitude and role toward ROK and US alliance today. This article verified the alliance prevents North Korea from provoking, keeps the balance of power in Northeast Asia, and supports prosperity and security in ROK, while it turns to value of alliance by various data. Therefore, the article suggests that ROK should develop national power to defend itself, make a solid ROK-US alliance, and make a firm public support to the alliance.