The US, then would probably go for either Liverpool (lots of Irish in Liverpool means perhaps pro-Irish sentiment?) or somewhere in Wales.
To successfully stage a Sealion, the US would need to get their hands on a really big port opposite of Ireland. "Somewhere in Wale", as beautiful as the scenery is, won't be of much help.
Liverpool is the scenario. I imagine the invasion to happen on the beaches of Blackpool and Southport and the US forces would go South and Southeast from there towards Manchester and especially Liverpool.
The only alternative objectives are not inviting at all, as they are rather deep inlands, sending every shipment through long sounds where enemy outposts might cause trouble for time to come (see Antwerp). This cancels
out Glasgow as well as the Swansea/Cardiff/Bristol-region.
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However, I am not sure if the UK wouldn't be able to prove tougher than Japan could have been. More industry, a bigger navy, probably a more modern Air Force. I have serious doubt if the US are able to get their hands on Ireland if not in a Pearl-Harbor-like surprise scenario with Dublin's friendly invitation.
But, I deem it a 99% certainty, that in the way up to such a conflict, something must have happened concerning Canada, which would warn Britain just as the occupation of Prague had been a wake-up call.
Forces in Ulster would be prepared to move in this timeline.
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However, though Ireland looks like the direct route, someone with more knowledge about the place should tell us about the possibilties. Does it possess the ports to make it the staging ground for destroying the British Empire? I would also point out that Dublin and Belfast are in a vulnerable position opposite Britain.
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The more I think of it, the more it ressembles an ordinary sea mammal of the unspeakable kind, unless, as others pointed out, the US find Allies on the mainland - either France or a Germany which controls enough coastline to allow a strategical connection with America via Norway or Northwestern France; or both countries united.
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Far cheaper idea: just undermine the Brits as in "The Ghostwriter".
