Cornwall is part of Britain. It is the westermost end of the peninsula that forms the far southwest of the island of Great Britain. Today, it is one of the counties of England, but it first appears in history as part of the area controlled by the Dumnonii, one of the ancient Celtic tribes of Britain in the times before the coming of the Romans.
The period of Roman domination seems to have had little effect on Cornwall, and it became a part of the kingdom of Dumnonia after the departure of the roman Legions. After the invading Saxons crushed Domnonia, Corwall survived as a separate kingdom. The history of its absorption into England is very unclear, but by the time of Edward the Confessor, who came to the English throne in 1042, it was an English possession. The native Cornish landowning class was quickly disposessed of its holdings, but the Cornish peasants kept ther language and culture intact for several more centuries.
The beginning of the end of the Cornish language came in the aftermath of a failed rebellion in 1549. England had mandated that church services be Protestant and held in English, and the failure of the resulting rebellion in Cornwall and Devon meant that English was to increasingly dominate the Cornish linguistic landscape. By the end of the 18th century, the Cornish language was spoken only by a tiny minority, and was in danger of vanishing completely. This tiny minority preserved the language through the 19th Century until the arrival of Cultural enthusiasts such as Henry Jenner, who published a book in 1904, "Handbook of the Cornish language", which started a revival that has persisted to the present day.