Conversation so far:
What were the historical origins of the Bretons?
The Bretons trace much of their heritage to groups of Brittonic speakers who emigrated from southwestern Great Britain, particularly Cornwall and Devon, mostly during the Anglo-Saxon invasion of Britain.
What is reason to believe the Breton's origins?
The Bretons migrated in waves from the 3rd to 9th century into Armorica, which was subsequently named Brittany after them.

Question: what is the most important fact in this article?
Self-contained rewrite: What is the most important fact about the Bretons?