This is a standard game mechanic. Your forces may not hold allied provinces. The enemy will always capture the province point first and not engage the third parties forces that are there.A German Group is unable to defend another German's Group capital against an enemy .
The capital is falling whatever be the allied forces inside.
At that point both armies will be sitting motionless in the province waiting for the other to make the first move. This is a game of "flinch" meaning that whichever army moves first will attack the other.
In reply to your statement that "this is a standard game mechanic", I don't understand how that can be. This map is based on the team game mechanic, in a sense, or so I thought. And I thought in a team game that your ally can't lose their province (be it a capital or just a normal one) to an opponent if your own forces guard it in place of his own forces.
That's supposed to be the job of an ally, to protect against all aggression, so that your forces do act like a team. If that is not true in this game, then you should be allowed to capture your ally's province for yourself rather than simply letting your mutual enemy take it right under your powerful army's nose and without a fight, even.
PS: I also have a few things to say about this map in general. Please take note of my sincerity in dealing with the uncomfortable subject to be found within.
This map is nice but so imbalanced. I'm in a match as UK with active allies. Yugoslavian Germany is inactive but the other two Germanies are active. Only one Soviet is active. Anyway, the other two active Germanies are kicking butt and taking names. I'm all but beaten in North France, so too is the Americans. What little that France started with is trying to survive but the game is all but lost. The East Germany is taking over all of the Soviets and also kicking us in the crotch in Italy even though the Yugoslavian Germany is inactive. The West Germany used Gold to buy a bunch of submarines and took out one of my Battleships so I can't even bombard the German army in Netherlands.
I know Gold is allowed, and I know it sucks when other players go inactive, but such is life. But my team is all active and yet we're getting totally trounced by just two players who are also taking out the three Soviets. And the Soviets have one active player also. I don't understand why they'd make the three big stacks of Germany in east and west so ridiculously big and inside a strong fort, too. It is impossible to beat. Yet, it is only day 4 and the Germans have nearly won the match already, taking most of Europe like lightning.
I think the German people at Bytro might have made this map so that their heritage of losing can be seen as not fair, as if Germany was much stronger than they were at this point in the war, and just to relive the glory of Germany's past instead of the shame of Germany's past. But Germany was losing badly on all sides and they were running out of resources, manpower, and allies. There's just no justification for this revisionist history purported to be a "balanced" map.
In fact, this map is not balanced. In fact it is imbalanced in favor of Germany whereas the true history would have it be imbalanced in favor of the Allies and the Soviet Allies. The lack of playable balance is unsettling, but the total revisionist view of the war at this stage is completely vomit-worthy. This map wouldn't be garbage if they admitted it was a "what-if" map, like the Clash of Nations map. But they pretend that it accurately reflects the true history of the war at that point in time and it does not. That is why I deem it "vomit-worthy".
Now, this map seemed like a nice idea, and perhaps a good fight. But my experience tells me that it is a terrible map, not worth playing, because of the lies. If they fix the balance, fix the resources to match what each nation realistically had (both available and rates of income), and fix the terrible terrible lack of research (which should also be balanced to each nation correctly), or, at the very least, admit it is not a historically accurate map, then, and only then, would I ever consider playing this map again.