Aragorn said
Explain the American revolution. Underdogs ftw.
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lrn2history
I was gonna write out a huge post explaining why, but I don't want to get into a debate with all the 'murrikans here. It suffices to say that a colossal portion of the rebellion's victory was to do with luck, and to do with the aid of foreign nations who wanted to mess with Britain (the French in particular, as well as the Spanish and Dutch, all sided with the US at various points, if I recall correctly). And also, I think, to do with the fact that (particularly, but not solely, at the beginning of the war) the British (or, rather, Howe) were not willing to go all-out. Had they been, the rebellion would have been crushed very early on.
Although thinking about it, I prefer the question "were the rebels actually the underdogs?" more than "could the British have beaten them?" That'd be much more fun to argue. >:3
EDIT: Not that I'm claiming to be an expert on the Revolutionary War. I've never studied it in detail; my opinions on it are based mainly on what I've read around the internet from historians every single time this argument comes up.