If the USA hadn't entered WW1 (which Wilson explicitly promised not to do in the 1916 campaign), Europe would have had to find its own way out of that mess. Which would probably have been better for everyone.
Harry Browne speculated that without America's entry into the war, the parties may have negotiated a peace in 1917, because America didn't tip the scales and give France and England hope for a total victory. Germany got out of it with their dignity intact. They never had to ship Lenin to Russia to stir up trouble, meaning no Bolshevik revolution, and no Stalin. Without a cause, Hitler would have lived a quiet life doing something useful, like hanging wallpaper. World War 2 could have been avoided.
Just speculation, I know...