In 1856 Democrats united behind Doughfaced Northerner James Buchanan and so kept Republicans as the minority party.
In 1860 Democrat Fire Eaters made certain that could not happen again, split their majority Democrat party and so gave victory to Lincoln's minority "Black Republicans".
Democrats then used Lincoln's election as their excuse for secession and war against the United States.
In 1912 the situation was reversed -- Teddy Roosevelt split his majority Republican party and so elected Southern Democrat Woodrow Wilson.
Wilson stayed out of the First World War as long as he could, but arguably, had Wilson remained neutral, then Germany would have won that war and so likely there would be no Second World War, certainly not with Corporal Hitler in charge.
But is that really the world you'd want -- Confederate slavers rule over the Southern Golden Circle, while the old Kaiser-Reich rules over continental Europe?
Finally, in 1964 Republicans effectively split over civil rights, giving a landslide victory to Texas Democrat Lyndon Johnson.
Johnson, responding to Republican criticisms of "weak on Communism" in Vietnam expanded the American military role there.
It's not at all clear that, had Goldwater won in 1964, he would have done substantially different than Johnson did in terms of numbers of troops, though perhaps with somewhat different tactics.