Soviet union signed a non-aggression pact with Germany before the invasion of Poland in 1939, and in fact they honoured such pact up to the begin of the Nazi Germany hostilities. Unfortunately, Joseph Stalin was confident that the total Allied war machine would eventually stop Germany, deploying an insufficient military power which sistematically lost territory. Soviet Union finally stop the German forces advance at 30 kilometers from Moscow, finally taking the initiative.
Fast and cheap, this is as close to a zerg as you're gonna get. You can undeploy any gun, including AA guns and late game raiding is something done almost exclusevly by soviets due to the fastest tank in the game being also one of the cheapest. Cheap infantry means a loss is not such a big deal, since more can be made really fast. Soviet weapons are known to be the most accurate ones, especially anti-tank weapons, giving a much more reliable solution to enemy armor then other factions. Soviets also possess the cheapest buildpower, at about 330 per constructor, which can also cloak to hide from enmy units and infiltrate enemy bases, possible making mines, barracks and other nasty suprises. When soviets get raided, they don't rush their AT guns to the storages - they rush the storages to the AT guns. As the faction with the only mobile storages in the game, you can never truly know where you should attack to make it hurt. And if you don't catch a soviet by suprise, the supply truck will just outrun most invaders.
While having speed and good aim helps, soviet guns are generally weaker and not as well armored as enemy equivelents. Infantry captures flags at a slower rate (about 10 times slower, meaning a squad of soviet infantry will capture flags as fast as a single rifleman). This is compensated by having commisars capture 10 times as fast as normal infantry, forcing you to constantly make a decicion: to build, or to expand. Soviets are always slower to get tanks, as they do not start with normal builders like others, but rather basic builders and making a barracks before able to do anything else really slows the soviet start. If soviets chose to be mobile, undeployed trucks are vulnerable to fighters and intercepters, as they can start attacking before entering the ranges of your AA, easily popping the ammo supply.