Well that's a great question. For starters, even before the 19th Century, indeed even before the US became a nation, there were Europeans and European-Americans who defended indigenous rights. Bartholomew de Las Casas is probably the earliest case -- he's definitely the best known early defender of indigenous rights. He wrote very critically of Columbus.
Before the Plains Wars, but already in the 19th Century, the Indian Removal Act did not have universal support. The leading critic of the Indian Removal Act was John Ross -- Ross was one eighth Cherokee. Jeremiah Evarts, co-founder of the American Board of Foreign Missions -- an organization for Christian Missionaries -- was very critical of Indian Removal, as was US Senator Sprague of Maine. It's worth noting that the Indian Removal Act itself only passed the US Senate by a vote of 28 to 19, and passed the House on an even narrower vote -- 102 to 97. Although most Representatives from the Southern and Western states were supportive of the legislation, a leading opponent was none other than Tennessee Congressman Davy Crocket (the one with the Disney TV show who later fought at the Alamo...)
None of this absolves the United States for Indian Removal or its actions during the Plains Wars. (Your initial question was about the Plains Wars which I will get to momentarily.) If anything, seeing the past as an inherently backwards time when harmful things like Indian Removal were inevitable byproducts of antiquated perspectives does much more to absolve the people who did deliberately choose to implement those policies.
Okay, and now for the Plains Wars. After the Sand Creek Massacre in 1864, there was a humanitarian backlash, and Congress appointed the Doolittle Committee to investigate US Indian Policy. Unfortunately, the Doolittle's recommendations, later adopted by President Grant in 1869, only encouraged a more humane paternalism that was still geared towards assimilating Native Americans into US society, rejecting traditional customs, wearing European clothing, etc. Similarly, in the 1880s there was another push for a more humane treatment of Indigenous peoples that culminated in the Dawes Act/General Allotment Act (1887). This reform, which was largely made with humanitarian intentions, nevertheless had the effect of halving the amount of land owned by American Indians on Reservations. The reason that this happened was that the Dawes Act made it so that land that was shared communally on Reservations would be split up and allocated to individuals as private property. Since US Law did not allow women to own land as private property, only Native men were granted parcels of land.
What about groups? In London, a group of British Quakers helped found the Aboriginal Protection Society in 1837. This group advocated for the sovereign rights of indigenous peoples around the world, but it wasn't perfect. One person who was involved in APS efforts in the 1840s was the infamous John Eyre (who brutally suppressed black Jamaicans in the 1860s). The APS was in favor of protecting the sovereignty of indigenous peoples, but it also believed that indigenous peoples should be "civilized"... So in short, there were always critics of US and European colonial maltreatment of Indigenous peoples, though that criticism of policy/defense of indigenous rights certainly had it's own limits...