The longest war in U.S. history is generally considered to be the War in Afghanistan, which began in October 2001 and continued until the final withdrawal of U.S. troops in August 2021, marking nearly 20 years of military engagement.
Despite its duration, the Cold War, which lasted from approximately 1947 to 1991, is not classified as a traditional war. The Cold War was characterized by geopolitical tension, ideological conflict, and a series of proxy wars, rather than direct large-scale military engagements between the United States and its main adversary, the Soviet Union. There was no formal declaration of war, and U.S. military forces were not continuously engaged in combat within a specific theater as they were in Afghanistan.
The nature of the Cold War focused more on political and economic strategies, including different forms of warfare like psychological operations and propaganda, rather than outright conflict. As such, while the Cold War spanned a significant period, its lack of direct military engagement between the U.S. and the Soviet Union is why it is not considered a traditional war and thus not the longest in U.S. history.