I take it you mean the Plains? Well, they fought each other all the time and some became embroiled in the Riel Rebellion. I think the major factor was that settlement was later and more sparse and then the buffalo disappeared leaving the tribes dependent on the government. Relations with the rather more diplomatic NWMP (who were, initially at least, seen as a benefit by the Indians because they made it their business to target the sale of liquor from Canadian and US 'traders' who were causing considerable damage to the tribes)while often painted more rosy than they actually were, also helped.
As far as fighting each other is concerned, it happened all the time and well into the reservation period Crows from the US were raiding Canadian Blackfoot for horses and vice versa.
Really well put grahamew! But let us also remember the East. I personally visited the battlefield near Thamesville, Ontario, where the great Shawnee chief Tecumseh was killed. This was indeed a real battle much like the ones in the US at that time.
Everything happens for a reason that only the Great Spirit knows and we wonder about.