Complex sentence: During the winter, Charles captured all of Eastern and then in the spring most of Western Brittany, leaving only the tiny port of Brest in the hands of Joanna of Flanders and a few English adventurers led by Walter Manny.
Split into simpler sentences: During the winter, Charles captured all of Eastern Brittany, and then in the spring most of Western Brittany. This left only the tiny port of Brest in the hands of Joanna of Flanders and a few English adventurers led by Walter Manny.