What is the longest war in u. s. history?

The longest war in us history is The Vietnam war

The longest war that the U.S. has participated in was the Vietnam War

The Vietnam War started in November of 1955 and ended in April of 1975, which resulted in a total of 19 years and 5 months of fighting.
 
The second longest war is the War in Afghanistan, which is still ongoing since October of 2001, resulting to a current 15 years and 6 months. Maybe it will top our conflict in Vietnam in the next five years if the fighting continues. Anyways, just thought I'd throw in a little extra fact on ya ;)

Thanks for the question! I hope this helped! Have an amazing rest of your weekend and feel free to let me know if you need anymore help! :D

They weaken the south vietnamese army.