As far as I understand, the reason why the second "Red" revolution occurred in Russia was because the new representative government had promised to end the war, but the Western allies could not let them lest Germany be reduced to a one front war. One of the reasons why the United States waited so long to go to war was because Wilson couldn't sell the war as a "war for democracy" if they were allied with the most backwards, czarist regime of Russia. The fact is, the Russians were doomed to revolution, irrelevant of the entry of the United States. Both the White Russians and Red Russians wanted out of the war. It's just the Whites were dependent on foreign aid from allies to stay in power.
The true question for Russia, therefore, is whether or not a serious, organized attack against the Bolsheviks following World War I would have saved the Russian republic during the Russian Revolution (lasted until 1922, if I remember correctly).
Also, the Spring Offensive by Germany in 1918...the infamous strike that gave the illusion that the Germans were going to win the war, despite their equally severe morale and domestic problems at home...this was a desperate effort to finish the war before the United States forces had been deployed. More than likely, if the United States hadn't entered the war, the Spring Offensive would not have worked out the same way...and then, you really can't predict what would have happened.
And, in terms of a world leader...well, take a look at Europe. They were devastated by 4 years of slaughter. The United States lost relatively little, so I would say they would still be in a powerful position. It's just like the Civ game where two superpowers beat each other into submission, and then the second rate power that wasn't close to the top 40 years ago is suddenly the victor by default. Also, look at the casualties to the working class...the United States lost relatively little of its population of males in the 18-30 range. Take a look at the European workforce in 1913 and 1918. Not pretty, is it?
EDIT: Typo!