If anything it was America that pioneered the END of the stealing of land from a conquered opponent. Instead, America rebuilt conquered foes into formidable trading partners with democratic freedoms and healthy economies.
The spoils of war were equally acceptable in 1850 as they were in 10,000 B.C.
You ignore my entire point that the North American Indian tribes claim some sovereign dibs on the continent as if they blossomed on Midwestern prairies. American Indians' grievance their land was violently stolen from them, which of course I do not dispute, is not mutually exclusive from the high probability that they stole it from some other people as well. Same goes for the Mexicans. We stole it fair and square and we don't owe anyone any apologies.
Define "stealing fair and square".
I will say that stealing "fair and square" was losing its legitimacy by about 1900. 10,000 years ago, "stealing fair and square" was quite honorable and continued to be honorable for thousands of years afterward.
Also, it's easy to say "stealing" is ok when one is the thief. If you are the victim, there is a greater tendency to say stealing is wrong.