The young William entered the navy in 1755 aboard the 80-gun bound for North America in the fleet of Admiral Edward Boscawen. Cornwallis was shortly after exchanged into and was present in her at the siege of Louisbourg in 1758. The siege was one of the pivotal battles of the war. Louisbourg was the only deep water harbour that the French controlled in North America, and its capture enabled the British to launch an attack on Quebec City. General James Wolfe's attack on Quebec and victory at the Battle of the Plains of Abraham saw the beginning of the end of French colonisation in North America.