Complex sentence: Historically, the North Sea has featured prominently in geopolitical and military affairs, particularly in Northern Europe but also globally through the power northern Europeans projected worldwide during much of the Middle Ages and into the modern era.
Split into simpler sentences: Historically, the North Sea has featured prominently in geopolitical and military affairs, particularly in Northern Europe. It was important globally through the power northern Europeans projected worldwide during much of the Middle Ages and into the modern era.