Complex sentence: In the 16th century, the Isles of Scilly were ruled by the King of England, forming an important strategic location on the route between mainland Europe and Ireland and Scotland, but threatened by the forces of France and Spain.
Split into simpler sentences: In the 16th century, the Isles of Scilly were ruled by the King of England. They were an important strategic location on the route between mainland Europe and Ireland and Scotland, but threatened by the forces of France and Spain.