At the heart of this poem—and at the heart of all colonial and imperial thinking—is that those who lived life differently than the way white Europeans and Americans did were lesser people. They were seen as less civilized somehow, and that often meant that they were described as wild, savage, or (only slightly less insulting) somehow closer to nature. Of course this is a ridiculous notion, one that's clearly racist by our own modern standards. At the time, though, colonial takeovers were seen as favors to the inhabitants whose lands were taken over. They got the gift of "civilization," and all it cost them was all their natural resources, their traditional way of life, their religious beliefs, their language, their dress… oh, and don't forget about all the death and destruction that often went along with these invasions. Gee, thanks.
Line 6: In addition to being called "wild," the native peoples in this line are described as "fluttered folk." The image here is of a butterfly, moth, or small bird—something frivolous and untamed that would be hard to contain.
Line 8: A "half-devil" is certainly not something you'd be likely to come across in the wilderness (we hope not, anyway), but this metaphor is pretty much as far from the idea of "civilization" as you can get.
Line 16: Let's leave aside the paradox of "wars of peace." The idea that these wars would be "savage" seems to suggest that a) either they're fought against savage locals or b) that they're in and of themselves an uncivilized means of achieving a civilizing goal.
Line 23: "Folly" here is personified to explain how the locals will frustrate the white men's attempts to complete their civilizing work. The fact that Folly is "heathen," or uncivilized, reminds us that any lack of progress is the fault of the local populations, not our sophisticated white guys.