From today's standpoint, we would say they were not.  But by the standards of the time they were certainly justified.  These days, we think that it is wrong to take control of other countries simply because we can.  But in those days that was pretty typical of what all countries did.  When the US was being imperialistic (as in the Spanish-American War) they were just doing what every other country with any real power did.  Therefore, their actions were completely justifiable in terms of contemporary thinking even though we would disapprove of those actions today.