At The Battle of the Rosebud, General Crook was surprised by the numbers and fighting style of the Indians who attacked him. To understand this you have to put aside all those Hollywood images of Indians swarming over the hills to attack the wagon train.
Until they began fighting the US Army, Plains Indian war parties tended to be much smaller. Large parties of warriors stirring clouds of dust were easily spotted by enemy villages and large numbers of men with two ponies apiece would be hard to sustain. Instead, warfare was more often conducted with lightening-quick strikes by small raiding parties. Also, fighting among Plains Indians was closer to sport than warfare, and, except for raids for blood vengence, killing was not the main goal. Instead, they gained honors by "counting coups." The most signficant coup was to touch a live enemy.
Though there were other rolls for men such as spiritual leader or history keeper, accumulating honors in battle was the main means for young males to earn their place in the tribe, and because of this they tended to fight for individual honors rather than as a disciplined group. There might be some strategy in the timing of the battle or setting up an ambush, but once the battle started it was every man for himself, and more often than not some young man started too soon and gave the ambush away.
When he killed the enemy a warrior would stop to take proof of his prowess, usually a scalp. It is important to note that scalping normally took place after death. In the rare instances where someone was scalped alive it was usually because they were unconscious or playing dead.
Once Lakota and Cheyenne warriors started coming up against the white man's disciplined armies, observant leaders like Red Cloud and later Crazy Horse began to realize that a more efficient fighting force was required. Prior to the Rosebud, the worse defeat for US troops was the Fetterman Fight in 1867 where 80 men were annihilated after being decoyed into an ambush of 1000-1500 Indians.
However, such large war parties remained rare and Indians continued to fight in the same manner, for individual glory and stopping to count coup. By the time of the Rosebud, however, Crazy Horse had learned the necessity of fighting in large numbers and a more unified force. He discouraged his warriors from stopping to take scalps for individual honors, which took valuable time away from the fight and exposed them to enemy bullets.
The results set the stage for Custer's defeat at the Little Bighorn.