Silly question, really, because a war is NEVER like a boxing match or a football game, in which the rules and conditions are always the same, and you can make an intelligent prediction of the outcome based on the combattants’ relative strengths.
Wars are fought under drastically varying conditions in all kinds of locales, for all kinds of reasons. How can anyone say “The U.S. would win” or “Britain would win” without knowing (aomg other things):
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Where’s the conflict to take place?
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What would each side be fighting for, and how badly do they need to win?
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Which side has more forces on hand where the real fighting will be done? How easily can they be reinforced?
There are many other questions, of course, but those will suffice for now. Fact is, it doesn’t always matter how much bigger one side’s army or navy is. Sometimes, it’s a question of priorities. Other times, it’s a matter of location.
Is the U.S. much stronger than the U.K. today? Of course it is, just as the U.K. was much stronger than the U.S. in 1776. It doesn’t follow that the U.S. could successfully conquer the U.K. today, any more than the U.K. could simply crush the American Revolution. Launching an invasion across the Atlantic Ocean is no easier now than it was in 1776!
IF the Brits had seen fit to use all the might at their disposal to crush the American Revolution, there would be no U.S. today. ANd if the U.S.A. today wanted to conquer and subjugate Britain, we probably could… but in each case, the end result would be of dubious value, and would require a MUCH greater expenditure of money, manpower and lives than either side would ever be willing to make.
Point is, even in 1776, the relatively weak U.S. was capable of beating the British Army in some locations, under some circumstances. And even now, the relatively weak British forces could beat U.S. forces in some locations, under some circumstances. Situational logistics are key here.