Answer:
It was justified on the vision of the Manifest Destiny doctrine, and the civilizatory mission of the American nation.
Step-by-step explanation:
The acquiring of Native land was justified primarily on Manifest Destiny, that is the notion that American would bring civilization to the unruly and savage frontier. Additionally, this cultural point would be strengthened with the economic notion of progress by using the "underdeveloped" resources in those lands, together with the religious one of expanding Christianity.