What was the longest war the US has ever entered?
The longest war the United States ever entered was the Vietnam War.
The second longest war in U.S. history to date was the War in Vietnam. In June 2010 Operation Enduring Freedom(The War In Afghanistan surpassed the Vietnam War for the longest in US History. If you count the "Cold War" as a war then Operation Enduring Freedom would be the second longest war in US History.