I'm very much just getting into this system, but based on one limited experimental run of the early part of WW2 via 1939 Storm over Europe/SC2 it seems things are guided via the Event Scripts along a very linear historical path.
I was running Germany in the scenario that starts March 1939 just to become familiar with the Research section, but on the side I also decided to forgo the pleasures of invading Poland and be a good european citizen. My reward for being a good little non-aggressive member of the great european community was for Russia to invade and occupy the Baltics and all of Poland. Then France declared war on me. Then Russia declared war on me. Then UK declared war on me. This, or at least the UK France part, is very hard to believe as happening. Knowing the inevitability of the Allies attacking forces Germany to be very aggressive and move forward in lock step with history, a history in which Germany can be criticized for not being very adroit. I don't like this.
Given the linear drive of the stock game, is it even possible to avoid US entry into either WW1 or WW2? Especially, in the case of WW2 it should be quite possible to avoid US European involvement.
However, this is the SC engine, so I can go in and mod the scripts to my liking. Hence, I like SC
Still, it seems the scripted events might have been more imaginative.