History of British vs American English

The English language was introduced to the Americans through British colonization in the early 17th century and it spread to many parts of the world because of the stength of the British empire. Over the years, English spoken in the United States and in Britain started diverging from each other in various aspects. This led to two dialects in the form of the American English and the British English.

American vs British accent

Prior to the Revolutionary War and American independence from the British in 1776, American and British accents were similar. Both were rhotic i.e. speakers pronounced the letter R in hard. Since 1776, the accents diverged but English accent in America has changed less drastically than accents in Britain.
Towards the end of the 18th century, non-rhotic speech took off in southern England, especially among the upper class; this "prestige" non-rhotic speech was standardized, and has been spreading in Britain ever since.
Most American accents, however, remained rhotic.
There are a few fascinating exceptions: New York and New England accents became non-rhotic, perhaps because of the region's British connections. Irish and Scottish accents, however, remained rhotic.
To be fair, both American and British English have several types of accents and there is no one true American or British accent.