I know some tribes like the Blackfoot/Siksika had territory that was sort of cut in half. How was this handled when national borders were drawn? What kind of pushback was there from the natives?
There was no law or settlements in the Western Canadian prairies until the Northwest Mounted Police, newly established in 1873, arrived at Fort Whoop-up in 1874. Up until then there was no law nor any federal protection for the Indian tribes living in the Canadian side of the border and the US Cavalry was unwilling to become involved in disputes across the border.
The NWMP was formed 1873. Before that complaints of murder and mayhem going unpunished were reaching Ottawa so Colonel P. Ross Robertson was sent to Edmonton to investigate. He learned of 3 German families slaughtered by the Bloods, of a half-breed who brutally murdered his wife outside the gates of the fort and severed the sinews of an old Indian woman's arms who starved to death as she was unable to work. His report tells of a notorious Cree who was responsible for many murders and states that in 1871 some 88 Blackfeet were murdered as a result of drunken brawls. On top of this he says that the Indians code of honour includes murder and assassination which caused another untold number of deaths.
This report, "A Report of a reconnaissance of the north-west provinces and Indian territories of the Dominion of Canada," got the attention of the Canadian government. Once word of The Cypress Hill Massacre reached Ottawa they acted and formed the Northwest Mounted Police. It's purpose was to protect the Indians, stop the liquor trade in the area and we're also ordered to bring justice to the West. They arrived in Fort Whoop-up in 1874 and quickly earned the respect and trust of the local Indians.
I'm currently reading The Range Men by L.V. Kelly. It provides us with some of the history of the Indians, fur traders, ranchers and settlers that lived in what is now Southern Alberta. In his book Kelly tells us of Fort Stand-Off which got it's name from a group of whiskey traders who were running from the US Cavalry. When they crossed the border they stopped and turned around to taunt their persuers, who had to turn around and head home empty handed. Kelly goes on to tell us that while they respected the laws on the Canadian side of the border ,after the NWMP arrived, young warriors seeking fortune and adventure would cross the border in search of enemies to kill and horses to steal secure in the knowledge that they would be safe from prosecution once they returned to their side of the border.
Kelly goes on to say that the Blackfeet went to war with the Kootenay Indians in 1871. They were losing badly so set the prairies on fire in order to escape and force the Kootenais back to the mountains. This fire also chased the buffalo South across the border creating a shortage of food that winter. The buffalo were prevented from returning by the Sioux and were never to return to Canada.
Peter Erasmus was an interpreter for Methodist missionaries in early Alberta in his youth then went to on to be a trader, miner and guide for such events as The Palliative Expedition. His memoirs are recorded in the book Buffalo Days and Nights. In this book he tells us that he was hired by the Cree to represent them as their interpreter because they did not trust the government to be truthful to them when they negotiated Treaty 6. He was present during the negotiations and while the chiefs were debating the terms of the treaty and whether or not to sign it. During the Chief's private debates Erasmus said that Big Child was the chief who swayed the others to sign the treaty. In his speech Big Child said that had seen how his people were treated South of the Medicine Line and that there they were vulnerable to the depredations caused by the white man and had no protections while the Canadians treated them as equals. With the mass herds of buffalo quickly disappearing he felt that his people would be better off accepting the government's terms and signing the treaty instead of rejecting it and becoming drawn into battle defending their lands against the onrush of settlers who would away from them anyways.
If this history interests you I would highly recommend that you read the books I mentioned and also get a copy of *Bear Child: The Life and times of Jerry Potts. * Potts was a scout and interpreter for the NWMP when they first arrived in the prairies. He led a very colorful life, being involved in battles against the Blackfoot Confederacy, hunting down and killing his mother's murderer and stealing horses from both sides of the border. He was known to be a crack shot with a pistol who one time got into a drunken argument with a friend whereupon they went into the street and proceeded to shoot the ends off of cigarettes hanging from each others mouths. He was also hired to be an interpreter during treaty negotiations but his lack of words were of little use. Once, after a Chief spoke a long flowery and elegant speech introducing his tribe Potts simplified it and told the negotiating committee "He's damn glad you're here."