Who was the major, major factor in ending WW2 in Europe? The US or the Soviet Union?

Can we safely say that the US and Britain would never have won the war without the Soviet Union jumped in? Or the other way? When I was younger, I was taught mainly about the US efforts of the war, but now with the infos on the net, I realize the CCCP contributed huge to the defeating of the Nazi. So what should be a fair verdict?