the majority of ships was sunk or fled tho. Only very few were actually captured compared to the total naval strenght. Germany did also scuttle as much ships as they could before they got overwhelmed. Ships and for that matter planes could easiely escape and join allied forces unless they werent sea worthy or ready to fly or even simple suprised by the enemy.
With tanks and guns it is a different story, since more often than not they were abandoned (even tho against their order) by their crews and left behind. Thats actually how the UK got hands on Tiger 131, which was standing abandoned on a hill in africa.
That stuff aside, they probably dont allow it for balancing purposes to begin with. Imagine germany seizing a rather big chunk of the french navy (including battleships etc) and also a good chunk of their air force. That could easiely snowball out of controll if germany just invades all of europe for example.
To be honest I feel like it should be more situation related like with Spain as I mentioned which in Spain's case could be made an event instead of a flat out mechanic. it just doesn't seem right for the Republican fleet to just scuttle its self If the Nationalist win their both Spanish and scuttling the whole Spanish fleet just seems weird. unless that actually happen in real life then I'll be satisfied. I feel as if I'd have heard about that though?
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