The US too. If you have historical focus on, you'll either go to war in 38 or 41, likely 41, so you'll have some time to mess around and send volunteers to Spain, to China and to Europe once that shindig starts.
EDIT: If you're playing the Soviets right, you should have no troubles by the time WWII comes to your door. By 1940, you should own Afghanistan, Iran, Turkey and Finland at the least, and should've spent the last 4 years hitting up the industrial focuses and building military factories all over the country. If you're really on the ball, you'll have the Arabian Peninsula and Sweden on lock as well by the time you get dragged in. Only annoying part is that Iraq and the Arabian states will sometimes pop in and join the Axis if you're unlucky. If you can sweep them fast enough, that may not be an issue though. I took Turkey in four days, so I didn't have any issue there (naval landings in Istanbul and paratroops in Ankara). The AI is gonna be focused up on the border, because you'll be massing troops there, so paratroops and naval invasions can knock those small countries out before they step up their diplomatic game (You're the USSR, you're not going to use your transports once the big war swings around). Finland didn't join the Axis when I did it, but I hit Finland very early. Sweden, the war was in full swing but Sweden didn't join either side for whatever reason.