Complex sentence: The Romano-British were originally a diverse group of Celtic (mostly or wholly Brythonic) peoples living, and usually fighting, with each other, first uniting when Roman troops, mainly from nearby german provinces, under Emperor Claudius invaded Britannia in 43 AD.
Split into simpler sentences: The Romano-British were originally a diverse group of Celtic (mostly or wholly Brythonic) peoples living, and frequently fighting, with each other. They first united when Roman troops, mainly from nearby Germanic provinces, under Emperor Claudius invaded Britannia in 43 AD.