The most epic "fails" in US military history were the various defeats brought on by arrogance the US military suffered against various Native American tribes of the American west...
Fetterman "Massacre" - Order: Lt. don't chase the Sioux beyond the fort's line of sight.
Action - Fetterman leads his men over a hill and out of sight.
Result - Lakota and Cheyenne warriors slaughter Fetterman's force and torture them in revenge for what was done to the Cheyenne at Sand Creek.
Little Bighorn - Order: Now, George, don't get gready. Same some for us.
Response - Like hell I will!
Action - Custer rushes the 7th cavalry ahead and then divides his forces into three seperate columns and orders all to attack a Lakota encampment from different directions.
Result - Reno's column attacks one end of the camp, but is soon repulsed (Reno is also rumored to be drunk during the fight). Bentien's column never makes it to the fight and has to rescue Reno's forces as they retreat on a hill some distance from the encampment. Custer's column is left isolated on a hilly area and is destroyed in its entirity.