Complex sentence: The population of Britain may have decreased by between 1.5 and 3 million after the Roman period, perhaps caused by environmental changes or by plague and smallpox (around 600, the smallpox spread from India into Europe).
Split into simpler sentences: The population of Britain may have decreased after the Roman period by between 1.5 and 3 million. This reduction may have been caused by environmental changes or by plague and smallpox (around 600 AD, the smallpox spread from India into Europe).