Complex sentence: John of Gaunt assumed leadership in France, but the English lost towns and territory, including Poitiers and Bergerac, and English dominance at sea was reversed by defeat at the Battle of La Rochelle, which undermined seaborne trade and threatened Gascony.
Split into simpler sentences: John of Gaunt assumed leadership in France but the English lost towns and territory including Poitiers and Bergerac. In addition English dominance at sea was reversed by defeat at the Battle of La Rochelle which undermined seaborne trade and threatened Gascony.