My humble opinion:
One of the problems with any conflict simulation about WW2 in Europe is that the players have knowledge of the historical facts but the real life nations and leaders did not. It is a real challenge for game designers.
Hitler did not expect his invasion of Poland to bring the UK and France into war.
Nobody expected France to fall quickly, but, in the game, this must be a likely result. In the games on WW2 in Europe that we have already, the UK and France go with a maximum "beef up" of France in order to make sure that history is not repeated. This can't be what regularly happens in the game, if the game is to simulate the actual conflict. In the game, France must have the same weaknesses as in history.
Hitler expected the UK to make peace after France fell. In fact, he was CERTAIN of it, and really did not prepare for any continuation of a ground war. Let us recall that even in the summer of 1941 during the height of Barbarossa, Hitler began to demobilize Germany's ground war focus and switch it over to an Air/Naval focus. So, obviously the continuation of the ground war past 1941 was unexpected.
Stalin was totally surprised by Barbarossa, but, this is an aspect of the real war that games have handled well.
Britain's plan to invade neutral Norway didn't happen only because Germany beat them to the punch -- that's another aspect of the war that's not easy to simulate in a game.
Finally, it is important to address the issue of the German war economy. We know that in real life, it is arguable to state that Germany NEVER put itself completely on a war time footing, economic-wise. We know that the Gauleiters always kept for themselves significant war material and war resources for their own domestic use. Bormann worked very hard to make sure that Hitler himself never interfered significantly in this mis-allocation of war resources.
The German Nazi leadership made very clear after Stalingrad that there would be "Total War". This implies that prior to Stalingrad, the German war economy was not on a total war footing -- and the facts of history bear this out.
How does a game designer handle this design challenge? It's a very tricky aspect of the game that should be addressed.
Thanks for your post Champagne and I would say that Strategic Command uses a pretty robust engine with a variety of events and triggers to help address many of the possibilities that can happen in game, and despite our very good understanding of what happened in history and our attempts to avoid the same mistakes.
We use in game actions to drive various political alliances, war entries and even for some military mobilizations and avoid the pitfalls (as much as possible) of hard coded events or hard dates to handle key moments of the war.
For example, depending on German actions in the war they may delay Barbarossa a little bit, declare war on the USSR early, or they may eventually have to face a USSR at a time not necessarily of their choosing. Let's say Germany did not properly garrison the Eastern front or if they invaded the UK, the USSR may look at that as an opportunity to strike west under the right conditions.
Germany may go on a more total war footing and invest more heavily and earlier into Industrial Technologly, but of course this might come at the cost of investments in other key areas of research. We also use a bit of a catch up method when invested in Spying and Intelligence that can help to bridge the gap when one side takes a significant lead in particular areas of research that helps redress imbalances over the long term.
To help model the idea that Stalin was caught a bit by surprise by Barbarossa, we use a special unit event that places a historical OOB of units along the Eastern front, upon declaration of war, that are then typically destroyed by an initial Axis assault. This helps us ensure that a Soviet player doesn't pull these units back and away from the front, as historical hindsight would dictate, but at the same time, those units are then available to a Soviet player should an Axis player not properly plan and effectively launch Barbarossa and destroy these units as the Germans did historically.
There are inevitable liberties and some abstraction, but I think for the most part you'll be pleasantly surprised at just how flexible the game play can be with hindsight in mind, while at the same time working quite well within the historical framework of what one would expect for a European Theater WWII game.