THINK - Without the United States remaining united how could that have changed the outcomes for WW I & II? Much more for the worst of outcomes I fear even though Reagan and Trump may only be delaying the inevitable...
IF Germany had won the Great War, or not been subject to sanctions, then there would not have been a WWII. The US involvement kind of caused WWII in a way.
That's easy. Hundreds of millions of people would not have been killed. The US intervention in World War I was a disaster. It is probably the worst branch of history we could have chosen.
A United States of lesser power divided between the North and the South would not have intervened because it wouldn't have the robustness of power necessary to meddle in European affairs that were none of it's business.
Many and varied horrible tragedies would have been averted.