Complex sentence: During the European early Middle Ages, what is now Finland was a sparsely-populated area characterised by pre-state societies, which was gradually absorbed into the emergent states of Sweden and, to a lesser extent, Russia.
Split into simpler sentences: During the European early Middle Ages, what is now Finland was a sparsely populated area characterised by pre-state societies. It was gradually absorbed into the emergent states of Sweden and, to a lesser extent, Russia.