When European settlers first came to North America it was in a search for a western sea route to get them to India. They struck land after a long voyage west & conquered by use of the guns that were brought with them & at first inadvertently the un-resisted diseases brought with them. History tells us that when discovered to be also a weapon then the settlers so used these diseases. At first North of Mexico was a single country but later split in war to become USA & Canada. In fact the years between settlers & today are more complex in events than depicted but the gist is the same.
Canadian government provides a good link Indigenous by Canadian government.