Actually, I'm not sure it was an important or key event in world history. The Russian Revolution of 1917 (November I'm assuming?)
The November Revolution lead to the founding of the Soviet Union of course. It overturned the March revolution government, which was pro-West, pro-war, and pro Democratic. And throughout, roughly, the next half century, the USSR was the worlds major proponent and ally to Communist revolutions and governments.
During this time period, the Communist movement was at war with Fascist, Democratic, and capitalist countries. These conflicts were both "Hot" and "Cold".
Hot wars were places of violence , insurrection and invasions. World War 2, Vietnam, and the civil war in Greece are examples of these violent conflicts.
The Cold wars were contested in cultural ways, such as in the Olympics, the Space Race,
or in the arms race between the Soviets and Americans.
All of these conflicts lead to a lot of strife, expense and death during much of the 20th century.
However, Communism as a governing movement collapsed by the late 1980's. From both it's economic untenability, and internal cultural failings, to the pressure from the outside world it reached general failure. In January 1992, the Soviet Union itself disolved.
By now, totalitarian Communism seems like a historical dead end.
Therefore, while it was an expensive and deadly experiment in political theory during the 20th century, it may go down in the vast history of mankind, as not particullarly key to anything.
This is in marked contrast to the effects and theories of the American and French Revolutions which still seem to be alive and well in mankinds attempts to govern itself.
