Why Do Americans and Brits Have Different Accents?

In 1776, whether you were declaring America independent from the crown or swearing your loyalty to King George III, your pronunciation would have been much the same. At that time, American and British accents hadn't yet diverged. What's surprising, though, is that Hollywood costume dramas get it all wrong: The Patriots and the Redcoats spoke with accents that were much closer to the contemporary American accent than to the Queen's English.