Complex sentence: The North Sea was the centre of the Vikings' rise and subsequently, the Hanseatic League, the Netherlands, and the British each sought to dominate the North Sea and through it to control access to the markets and resources of the world.
Split into simpler sentences: The North Sea was the centre of the Vikings' rise. Subsequently, the Hanseatic League, the Netherlands, and the British each sought to dominate the North Sea and through their control of access to the markets and resources of the world.