The War to Prevent Southern Independence was a horrible, senseless waste of capital and human life. There were, in fact, only two unequivocally good things to come out of the war. The first, and most obvious, was the abolition of slavery — while it was certainly not the purpose of the war, and while no war was necessary to achieve it, the end of slavery was nonetheless a consequence of the war, and was of course an unalloyed good in and of itself. The other beneficial consequence of the war is that it took the United States off the path of empire it had been treading for fifteen years; the people were so busy with the attempt to rebuild a devastated and depopulated nation, and the would-be tyrants so busy attempting to establish and support the military junta assigned to rule over the conquered Confederate States, that scarcely a thought was given to foreign conquest, and, in the postbellum years, America no longer went abroad in search of monsters to destroy. All of this changed on 15 February 1898, when the USS Maine sank into Havana Harbor.
Presidential Rankings #26: William McKinley
The mission of the United States is one of benevolent assimilation.