@Lilmak
It usually is based upon the release date or first action of each plane. Remember that America, Russia and France demilitarized after WW1, but Japan and Germany and Italy researched and by 1930 had the best planes, and ships and making breakthroughs with tanks.
One of my western civilization professors once said that the allies only won WW2 by persistence and American industry. So if many different things didn't happen and America stayed out of the war even for 1 or 2 more years then Axis would of knocked out UK and China. With them out of the way the way would of been made clear for the conquest of USSR. For even with the Russians numbers If Japan, Germany and Italy, and by that point Ottoman Turks would of overwhelmed them and that would of been that.