Question

Was US expansion, during the 1800s, justified?

Answer 1

By the criteria of Americans of that day, who knew nothing about "political correctness," it was entirely justifiable.  They believed that their system was the best that had ever been invented and that it should be spread throughout the world without delay.  (This is one of the reasons they supported foreign missionaries and "filibustering" expeditions.)  The easiest way to spread it, obviously, was into contiguous regions, and thence into regions contiguous to those, and so on.  One Congressman stated that he hoped to live to see the day when the Speaker of the House would recognize "the member from Canada, from Cuba, even the gentleman from Patagonia!"