Unlike Britain and France, Russia generally supported the Union in the American Civil War, and the consequences of what would happen if any major European power intervened on the side of the Confederacy, would have been a bloody World War.
Russia and America, unlike today, were allies, and if Britain or France did enter, then America and Russia would crush them. Britain's army was only 50,000 strong in the 1860s, and it's navy was rivaled by America, who had around 500,000 to 1,500,000 troops(wikipedia). It would infact have been a walkover for America to defeat them. Infact, it would take an entire coalition of all of Europe's great powers(Britain, France, Prussia, Austria, Spain, and Russia) to defeat America, and of course, Russia would have fought with America.
This is true, as Russia was America's ally, and so it would probably be a quick war if Britain and France did enter.