Answer:
The poem is about the Christian expansion of the United States and its negative effects. Their "manifest destiny" was to make use of the land, which they did not believe the Native Americans or the other settlers (French, Spanish etc.) were doing.
The poem includes the Native American tribes among the nature that is destroyed (such as the buffaloes) by the expansion, because that was how Native Americans were viewed at the time. They were seen as "noble savages" who were part of the wildlife, and were about to die out anyway, so the belief was that they could be killed or oppressed without guilt.