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Without going full Sealion things go a little worse for the UK in 1940 and without being actually invaded decide they have no hope of defeating Germany in Europe and sign a treaty to stay out of it.
Japan is still doing their thing in Asia and still end up attacking the US in Hawaii, so the US is definitely going to war with Japan and Hitler may still declare war on the US after that. Hitler declaring war on the US IOTL was pretty illogical and wouldn't be much more so in this scenario.
So how does the USA wage its war on Germany without having a base to launch from in the UK?