Absolutely the idea of Manifest Destiny justified (or was used to justify) our westward expansion. We used it as a reason to move the Indians off the land and we, especially, used it as a justification for the war with Mexico in the 1840s.
The idea behind Manifest Destiny was, of course, that we sort of deserved the land more than everyone else. This was because we had a superior culture, a superior form of government, and were a superior race. This allowed us to justify removing the Indians (on both grounds of race and culture) and to justify taking land from Mexico (mainly on the grounds of culture, part of which was religion, and government systems).