The United States's biggest victory of the war of 1812 was that the British agreed to abandon it's attempts at creating and arming a unified Native American state in return for a promise from the Americans not to invade Canada again. This lead more or less left the door wide open for Manifest Destiny and sealed the fate of the Native Americans to be at the (lack of) mercy of the Americans. What are your thoughts?
Yes. I would phrase it slightly differently, though, and I have in fact done so in a previous post.
Quoting from that old answer:
One consequence of the successful end of the Indian war meant that, though the British never really articulated a particular policy as a result, they backed off from North America almost entirely. What this meant was that the Native groups could no longer rely on relatively balanced political and military power between the US and Great Britain, and the sole monopoly of political power in the Great Lakes region especially was the United States. It took a decade or so for this to become clear, but is unambiguously the major influence on North American history in the first decades of the 19th century. It is the basis for nearly every territorial decision with respect to Native territory and autonomy going forward.
There is a lot to be said about whether or not Great Britain actually wanted to establish a Native American state, or even if they did, if they would, but it was something that some British military and political leaders discussed or suggested. But there was a very important political change as a result of the end of the war, that indigenous actors in American territory or in future American territory could no longer rely on playing diplomatic tension between Great Britain and the United States. Having the ability to use the territory or resources of rival powers was a major factor in indigenous politics and diplomacy up until 1815, and afterward it wasn't viable.