A very real nightmare scenario could be Trotsky taking over all of Europe + the UK (during the 1920 battle of Warsaw, the dock-workers and other workers in England stopped work, refusing to ship aid to the Poles under the slogan "Poland, keep your hands off Russia")
and then he would have hit China and much of Asia
Trotsky was the one who SAVED the Bolsheviks during the Russian civil war -- the Bolshies had first undermined the Army by getting soldiers to kill their commanders, but Trotsky managed to pull off an incredible feat (and again, I recommend Zamoyski's book, it's short, but his words are superbly concise)
Trotsky was a military genius, but I think the chance of his conquering Europe if not stopped at Warsaw ranged from zero to less than that.
There was very little support among “the proletariat” of the nations of Europe for a Russian-led socialist revolution, and his invasion of Poland collapsed largely because of his distance from his base.