WW2 The German Empire could have threatened Britain
Indeed a German Empire could have threatened Britain but the overstretch that expansion into Russia caused makes me believe that this would have been unlikely and that Britain could have consolidated her strength and made the invasion process impossible. Because of the speed of any German expansion Alexander the Great's empire might be a better example and that collapsed soon after Alexander died. But dealing with counter factualism is pointless but I must point out that after WW2 Britain faced many more threats than she had before WW2 and was in a far worse position to face them. It is hard to imagine that WW2 was anything but a disaster for Britain, the important question being: Was the disaster of WW2 a lesser cost than not fighting? As to a Peloponnesian League, I would disagree. Any conventional diplomatic or military alliance is going to be easy meat for the US. Unions like the Delian league, the Warsaw pact or whatever give the US a simple geographical target which they can work on. Britain by dumping various treaties and trade agreements should be encouraging the technical development of regional powers like China which the US is not fond of to divert and divide the strength of the US. Your example of WW1 is classic although I think you overplay the importance of the US forces who entered the war after Germany had lost any chance of winning. Effectively they let the European powers fight it out and then when the time was right for the US they entered ensuring that not only did the powers they wanted to win won but that they owed a debt to the US. I do not see the US loosing to a great counter power but I do see an extended war against terrorism, a war with no goals or aims set in concrete combined with increased domestic concerns (crime, immigration, authoritarianism) that severely burns their fingers. the US is not all powerful and will eventually get hurt. The UK could if she shows sense benefit from this but if we stick to the US chances are we will get burned worse.