"Now that you're free of the colonizing mother country, you're going to stop acting like colonizers? Right...?"

Native American polities fought on both sides of the Revolutionary War, largely according to which side they thought would be least horrific. In the immediate aftermath of the war, the British ‘ceded’ large amounts of Native American land to the newborn USA, which didn’t help matters. That being said, the USA was not immediately horrific in comparison to the behavior of the British, though it was also far from just or fair by any reasonable or modern standard.

By the War of 1812, the increased rate of settlement and sharply decreasing support for co-existence or even assimilation with Native Americans in the USA led most Native American polities to side with the British, and by the 1830s, the US had developed a truly horrific standard of behavior towards Native American polities.