The British and later Canadian governments did not only have practical and policy reasons for entering into Treaties with First Nations, but they also had to consider domestic and international laws that recognized the rights of the First Nations, and recognized the Treaties as a legitimate way of dealing with these rights.
For a Treaty to exist, the representatives of the First Nations and of the Crown must have had the authority to make an agreement on behalf of their people. The Supreme Court has recognized that when the British came to North America the First Nations were “considered as distinct, independent political communities, retaining their original natural rights, as the undisputed possessors of the soil, from time immemorial . . .”5 Further, historical research shows that the British regarded the original occupants as nations, understood to be organized societies with their own forms of government. Thus, entering into Treaties with First Nations indicates that the Crown recognized the nationhood of the original occupants of what is now Canada.
Canadian common law, the Royal Proclamation of 1763, and other legislation have all recognized the rights of First Nations.
The common law is where the rights of First Nations developed. Common law recognized the existing rights of First Nations, including local customary laws and their right to occupy the land even after Britain began to rule what is Canada. Common law dates back to a time in Britain before there was a parliament to pass legislation. Judges would apply a common standard of rules to all cases heard in the country, with the rules originating from local customs. Common law rules continued to be laws even after statutes could be passed. Because First Nations had historically occupied the land, the local customary laws continued to apply even after Britain began to rule what is now Canada.
The right of First Nations to continue to occupy their lands was recognized by the British Crown in the Royal Proclamation of 1763. This Proclamation did not create new rights for First Nations but did recognize the rights of First Nations to their land, and the need for Treaties between the Crown and First Nations concerning the use of First Nation land. The Proclamation forbade British subjects from moving onto or purchasing lands occupied by First Nations. It also stated that if “Indians should be inclined to dispose” of their lands they could only be purchased by the Crown.
When the British Crown transferred what was called Rupert’s Land to the new Dominion of Canada in 1870, Britain required the Canadian government to consider and settle claims of Indian tribes for compensation for lands settled. As well, later legislation dealing with Crown land and opening up areas for settlement required that “Indian title” be dealt with before the land could be settled. Further, the British North America Act recognized that the Dominion of Canada had existing obligations to First Nations and that the process of dealing with the First Nations was ongoing.
5 R. v. Van der Peet,  2 S.C.R. 507. Quoting from the American Supreme Court case Worcester v. Georgia, 31 U.S. (6Pet.) 515 (1832).