1. maybe ''standard'' for the Army fighting the NAs = as many officers did not have a lot of experience in..it worked at the Washita because there were not many warriors.
You need to do a little serious reading on Custer's Last Stand. First off, there were plenty of warriors at the Washita. Second, dividing your force was a standard U.S. Army tactic when fighting against Indians. General Nelson Miles, a great Indian fighter, strongly defended Custer's actions at the Little Big Horn and argued that Reno and Benteen should have been prosecuted for gross dereliction of duty.
AND, a
US unit DID get massacred there because
they DID split up
2. it is a standard military rule--do not split up
Wounds from the Washita: The Major Elliott Affair
Again, you need to do some serious research. It was not a standard rule to concentrate forces when fighting Indians. For that matter, some Civil War generals, such as Robert E. Lee, achieved great success by dividing their forces during the war.
Custer held his own for
hours at the Little Big Horn
, partly because the Indians logically assumed that Reno's force would quickly be coming to Custer's aid, so they refrained from a close-up assault and were content to engage in a long-distance fire fight. But, when they saw that Reno's force wasn't going anywhere, they decided to close in on Custer.
Reno blundered horribly by leaving the timber (the sunken area of trees to which he initially fled after losing his nerve the first time). The timber was one giant natural rifle pit, and the Indians were shocked when they realized that Reno was making a dash from the timber to what would later be called Reno Hill. Reno lost one-fourth of his command in that foolish, blundering move. Also, by leaving the timber, Reno's force no longer threatened the village because Reno Hill was over a mile farther
away from the village!
I would recommend three readily available books on Custer's Last Stand:
Phillip Tucker,
Death at the Little Bighorn (Skyhorse Publishing 2017)
Nathan Philbrick,
The Last Stand: Custer, Sitting Bull, and the Battle of the Little Bighorn (New York: Viking, 2010)
James Donovan,
A Terrible Glory: Custer and the Little Bighorn (New York: Little, Brown, and Company, 2008)