[GECEDEANY]

The mini Ice Age that clobbered Europe between ~1600 and ~1800 and froze the Thames solid for months at a time on several occasions didn't seem to stop the British Empire from achieving global dominance.

Actually, Britain came right out of that and into what's been callled "Britain's imperial century" from 1815 to 1914.

Beyond that, I think that comparing Ammerica to the Roman Empire in any capacity is really little more than an exercise in pointing out (ultimatelly irrelevant) coincidences.