Before the arrival of the white settlers, we have little evidence relating to the Native People and warfare. No-one from a tribe had a reason to write, so no tribal 'histories' exist before white settlers started to move across America.
The knowledge that we do have indicates that the Plains tribes were relatively peaceful and that they only really clashed once the horse arrived. Horses were seen as a sign of wealth and power and the theft of a horse could cause much friction between tribes.
Warriors practicing for warfare on horseback
Even so, some tribes on the Great Plains believed that killing another tribe's men put too great a strain on those who remained in that tribe. Therefore, when fighting did occur, many Plains tribes used a system of coups. This was where you would ride up to an opponent and touch him with your 'magic stick' which had been blessed by the tribe's medicine man. This was considered a great sign of bravery whereas to be touched by your opponent's 'magic stick' was a great sign of shame which could only be redeemed by an act of bravery. Therefore, killings during tribal clashes were rare. They did occur but not with frequency.
The gun changed a lot of Plains warfare. Not only could guns be used in hunting buffalo but it could also give a tribe greater power over neighbouring tribes if they possessed them. Before the gun, Native People used the weapons of war most associated with them - tomahawks, bows and arrows, lances etc.
Ute warriors with rifles in 1874
A chief decided if a war with another tribe would take place. A war dance was performed and each warrior from that tribe had to swear that he would not leave his tribe during fighting. Once a war was over, both tribes would usually sit down together with the chiefs sitting "in treaty" smoking a pipe of peace. Each warrior who had fought would then smoke the pipe of peace and dance the pipe-of-peace dance. This was the scenario as witnessed by George Catlin.
Only two tribes were known to have scalped victims - the Sioux and the Crow - despite the attempts by 1950's Hollywood films to show otherwise. Most other tribes did not regard scalping as a sign of bravery as it took place after death. But even to the Crow and the Sioux it was important to their beliefs as it was believed that by scalping someone you had killed, you would be releasing his earthly spirit into this world and you would not meet that person's spirit in the afterlife as his spirit was trapped on Earth.