Complex sentence: Henry II of England invaded Brittany at the end of the 12th century and became Count of Nantes in 1158 under a treaty with Duke Conan IV. Henry's son, Geoffrey became Duke through his marriage to Constance the hereditary Duchess.
Split into simpler sentences: Henry II of England invaded Brittany at the end of the 12th century. His son, Geoffrey became Duke through his marriage to Constance the hereditary Duchess.