What mistake by Colonel George Custer led to his defeat by chiefs Sitting Bull and Crazy Horse at Battle of the Little Bighorn?

Answer 1

Flamboyant in life, George Armstrong Custer has remained one of the best-known figures in American history and popular mythology long after his death at the hands of Lakota and Cheyenne warriors at the Battle of the Little Bighorn.
And the word mythology really fits how he is remembered.

He graduated last in his class and then failed at his first posting to stop a fight between two cadets. He was court-martialed and was saved only by the need for officers with the outbreak of the Civil War where he did very well.

In July of 1866 Custer was appointed lieutenant-colonel of the Seventh Cavalry. The next year he led the cavalry in a messed up campaign against the Southern Cheyenne. In late 1867 Custer was court-martialed again and suspended from duty for a year for being absent from duty during the campaign. Again a friend saved him.

In 1876, Custer was scheduled to lead part of the anti-Lakota expedition, along with Generals John Gibbon and George Crook. He almost didn't make it because President Ulysses S. Grant relieved Custer of his command and replaced him with General Alfred Terry. Public opinion made Grant reverse himself.

He did have ambitions to be President one day. And he hoped that this would do it.
The original plan called for the three forces under the command of Crook, Gibbon, and Custer to trap the bulk of the Lakota and Cheyenne population between them and deal them a crushing blow. The troops of Crook and Gibbon were not at the same place with Custer as Custer was far ahead of them. Instead of waiting, Custer was so sure that he could win any fight with the Indians. He split his forces into three parts to ensure that fewer Indians would escape.

The attack was one the greatest fiascos of the United States Army, as thousands of Lakota, Cheyenne and Arapaho warriors forced Custer's unit back onto a ridge parallel to the Little Bighorn, surrounded them, and killed all 210 of them. Imagine thousands to 210. He just might have won if he had waited for the other two commanders.