One could pretty easily argue that European settlement in general, and American expansion, specifically, were unjustified.  The land was already occupied, and natives had a longstanding claim to North America based on where they had traditionally lived for centuries.

But we can't put historical events under today's moral and ethical microscope.  It was legal at that time, and few in society stood up and opposed it.  Understandable given the huge economic benefits to that expansion at that time.  We can expect humans in a similar situation today would also look for justification.

I think that the reasons for Westward Expansion were numerous. Some wanted to got west for adventure, some for wealth, and some were simply looking to find a spot they could call their own. As far as was the expansion done in a just manner, I would have to say no, not in all cases at least.

In today's terms, no, this expansion was not justified.  We no longer think that it is acceptable for countries to take land from other countries simply because they are able to.

But at the time that it was happening, there was definitely the idea that might made right.  The US was strong enough to take those lands and therefore it was okay for them to do so.

I do not believe that it is generally appropriate to judge people by standards other than those of their own day.  Therefore, I do not think that it was wrong for the US to do what it did.  (After all, for example, how did Mexico get the land the US took from them?  They stole it from the Indians, so why shouldn't the US turn around and take it from them?)