One of the differences between British and American spelling is between those words that end in -re in British English and -er in American English, such as fibre / fiber. In this post I look at five words ending in -tre / -ter - centre, lustre, mitre, sceptre and theatre.
Talking about the differences between British and American spelling at Oxford Dictionaries, they say:
The differences often come about because British English has tended to keep the spelling of words it has absorbed from other languages (e.g. French), while American English has adapted the spelling to reflect the way that the words actually sound when they're spoken.
Oxford Dictionaries
Which I understand to mean that these words entered English with their (for example) French spelling, and were later changed in American English. This change is usually attributed to Noah Webster, and especially to his An American Dictionary of the English Language, published in 1828. So I was rather surprised when I came across an instance of theater in a British book from the seventeenth century.