Despite their long history of conflict and rivalry, Russia and the USA have never fought in a war on opposing sides.
However, American forces have fought against Russians on Russian soil, and it can be (tenuously) argued that Russian forces have fought against Americans on American soil.
American forces spent a considerable amount of time on active service in Russia during the Russian Civil War. The American Expeditionary Force sent to Siberian in 1918 consisted of almost 8,000 men. Their main mission was to guard parts of the strategic trans-siberian railroad, and also to help facilitate the escape of the Czech Legion. The force suffered almost 200 casualties before it was withdrawn in April 1920.
A second American North Russian Expeditionary Force was also sent to Russia during the Civil War. Prosaically known as the Polar Bear Expedition, ANREF consisted of almost 5,000 US soldiers, and took part in a wider Allied intervention in the Russian war that saw them in direct combat with the Red Army.
As I mentioned at the beginning of this article, Russian forces have also been in combat on American soil. When Alaska was still a Russian colony, Russian soldiers fought in the Battle of Sitka, winning a decisive victory over the Sheet’-ka Kwaan people.
Russia and the United States have, despite their differences, been allies of convenience on more than one occasion. During the first world war, both fought on the same side until revolution led to Russia’s withdrawal from the conflict. And Roosevelt and Stalin worked effectively, together with Britain’s Churchill, as the Allies during the Second World War.
The reason that neither side has ever come directly into conflict with the other over the past 100 years or so is largely because the stakes would have been so high. During the aftermath of the Second World War, the Soviet Union had a dramatic conventional military superiority over the US in Europe, and there was real concern that the Soviet military would not stop in Berlin, but would keep on rolling until they reached the Atlantic. The risk to the Soviet Union of a draining conflict would probably have been too high, though, even if the US had not demonstrated its willingness to use the atom bomb in Hiroshima and Nagasaki. For the next forty years, the high stakes forced each side to content itself with fighting a cold war, often through proxy wars such as in Vietnam, Korea or Afghanistan.
The prospect of a Russia vs USA war continues to fascinate many people, though, and is a staple of fiction (particularly some armageddon fiction). It’s also a topic that never ceases to fascinate some conspiracy theorists, who remain convinced that Russian troops are invading US soil in their black helicopters.