A goof

Someone who knows how can enter this in the goofs sections if they want. No doubt it is a deliberate historical inaccuracy.

At a peace conference Crazy House says:

But the word of the great white father
is broken word.
Hear me now, Long Hair.
If this word broken now...
...not only Sioux, but the Cheyenne,
the Oglala...
...the Miniconjou, the Blackfeet,
the Sans Arcs...
...and every living tribe between
mountains and great waters...
...will gather in one last battle.
It will be the end of themselves...
...their gods, the spirits
of their fathers...
...and of their enemies.
Crazy Horse spoke.

The list of tribes is quite odd.

The Northern Cheyenne were close allies to the Sioux, and the Northern Arapahoe were close allies of the Northern Cheyenne, so naturally there were a lot of Cheyenne and some Arapahoe in the "Great Sioux War" of 1876-1877.

In 1800 there were seven tribes of Sioux: Mdwakanton, Wahpeton, Wahpekute, Sisseton, Yankton, Yanktonais, and the Teton or Lakota, who lived on the plains.

The Teton or Lakota then split up into seven tribes who all had Lakota names but are usually known by a mix of Lakota, French, and English names: Brule, Oglala, Sans Arcs, Hunkpapa, Miniconjou, Blackfoot or Blackfeet (don't confuse them with the Blackfeet or Blackfoot, a separate confederation), and Two Kettles.

So when Crazy Horse threatens Custer with the Sioux, the Cheyenne, the Oglala, the Miniconjou, the Blackfeet, the Sans arcs, that is like a US general threatening a foreign leader with the Americans, the British, the Pennsylvanians, the Texans, the Californians, the New Yorkers.

As for all the tribes between the mountains and the great waters joining the Sioux, the Sioux were more or less permanently and constantly at war with all of the plains Indian tribes except for their allies the Cheyenne and the Arapahoe. Hundreds of warriors from over a dozen plains Indian tribes fought against the Sioux in 1876-77.

In fact, many Sioux and Cheyenne agreed to fight the hostile Sioux and Cheyenne, which is one of the main reasons why the Sioux were NOT forced to move to Indian Territory in 1877 but remain in the north until the present.

The script has Custer and the Seventh Cavalry defeating Crazy Horse and the Sioux in a fictional long war some time after the US Civil War, implying that Custer could do much more with a single regiment then Sibley, and Sully, and Connor could do with brigades in 1863-1865.

Depicting Custer and the Seventh Cavalry as great Indian fighters who could defeat the Sioux created the problem that if the Seventh Cavalry was strong enough to defeat the Sioux unaided before, how could it be wiped out by the same Sioux at the Little Big Horn? If the odds were not hopelessly against Custer at the Little Big Horn he would seem like a fool to be defeated, but if Custer faced hopeless odds against the Sioux he could not have defeated them before.

Keeping the fictional plot element where Custer defeated the Sioux in his fictional first war with them would make his defeat by the Sioux at the Little Big Horn seem like he goofed instead of faced impossible odds. So the writers (Wally Kline, Aeneas Mackenzie, and Lenore J. Coffee) simply made all the tribes who were the enemies of the Sioux, and all of the Plains Indians tribes, the allies of the Sioux at the Little Big Horn to produce the hordes of extra warriors necessary for Custer to be defeated without blundering.