Simply great
I've seen quite a bit of flash and have played many games, and this game really gets almost everything right. The balance between plot and game play is sensible, and there is a good compromise made between the gritty aspects of the environment and the cartoon-like aspects.
Maybe I'm biased towards this game since I'm a little bit of a WW2 buff, but I think others will like your historical approach the way I did. You didn't try to be too dramatic or serious, but you also didn't treat the battle of Stalingrad like a joke. What could've made it better would've been an initial cut scene describing the dire state of Soviet Russia, and the bitter savagery of the German invasion.
That said, this game isn't perfect. There is too little mercy for failiure, which I realise is a design choice, but its simply frustrating to have no chance after the smg guy crosses a certain line. My suggestion would be to give your player a small amount of life. Not so much life that the realism and gritiness is lost (I actually really like how you don't make the player invincible, and have them react believably to being in the line of fire), but also make it so that it is possible to kill the smg guy.
In my experience, the difficult game done right is a game where it is hard, but not impossible to succeed when the odds turn against your favor. When a hard game is done improperly, its literally impossible to turn the game around. Rather than being stimulating, this scenario is frustrating.
So, in conclusion, this game has a laudable sense of style, game play, and originality, but still does not achieve perfection.