American destroyers being handed over to the UK is already in HOI3 as an event, don't see why it couldn't be in HOI4 as well

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That's what happened though. HOI 3 did a great job of somewhat following history with events, and from what we've seen HOI 4 is going to be AI instead of events, with goals leading the way.
So I'm hoping that if a German player is fast enough they'll get to capture France's (or Britain's?) ships, even more so that they're actually equipment now and are tracked.
The whole point of America giving Britain their destroyers was so that Germany couldn't conquer Britain because the idea is that if they did the only thing they'd have to offer Germany was Britain's fleet to spare them from complete annihilation.
Plus that would change the game significantly and a human America may want to build up the Atlantic fleet just like they did in real life so that they'd be able to fight the British (which has become German) fleet.
(Edit): For reference this navy thing has a previous backing when Germany conquered France, and although the French captains promised that they would scuttle their ships before they were captured.
Churchill wasn't assured by this and made a move to capture these French ships just after France falls, a prominent group of battleships wouldn't submit to the British and the result was the battle of Mers-El-Kabir, where Force H had to sink some of the French ships..
(I had just finished watching "Secrets of the Dead: Churchill's Deadly Decision" from netflix before thinking about the ships)