Im maybe making a poor assumption, but I believe we had a british accent back in the 1700s, since we were under British rule. But then 100 years later, during the civil war, it seems that the dialect/accent is gone. Am I right in thinking that we lost the accent in only 100 years or so?
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American colonists came from England, so, of course they had English accents (more than one, as they came from different parts of England).
(Britain did not exist as one country when the early American colonists first arrived in America. Scotland and England were both ruled by one King, but the countries were not united until the Act of Union in 1707.)
The English accents spoken by the colonists then were not the same as English accents spoken in England today. Languages and accents keep changing. Both American accents and English accents have changed in the past 400 years. Neither country speaks with the same accents that the original colonists spoke in England and then in America.
It is generally thought that the English accents in England have changed more over that period (due to a higher population and higher population densities) than the accents in America have.
Both, however, have changed considerably from what one would have heard spoken back then.
It is actually a lot more gradual than you may be thinking it. Accents slowly change, gaining new sounds, losing sounds, and other various phonological variations. So determining when the British accent became "American" is much like identifying when red becomes orange on a colo(u)r gradient. However with a working definition of what constitutes an American accent you could point to a specific time where the spoken English meets all the criteria but even then, it happens at an individual level so I wouldn't say it's something one could really say.
So many problems with what may seem a simple matter! And this is all assuming that there's such a thing as an American accent, when in reality its a smorgasbord of different ways of speaking English.
So in conclusion, you're kind of right... But don't see it as losing the accent; the Americans slowly changed what was spoken 100 years prior because of the inevitability of language change. Keep in mind, the accent you speak now will be different than the one spoken by your great-great-grandchild in 100 years and he could ask: when did we lose that accent?
Interestingly there are a few small, isolated communities in the US that have a particularly old accent. Tangier Island is known for it and might possibly be the closest to what the colonists actually spoke like. You can hear some brogue to it, but it was probably never "British" like we hear today.