The tide turned in 1757 because William Pitt, the new British leader, saw the colonial conflicts as the key to building a vast British empire. Borrowing heavily to finance the war, he paid Prussia to fight in Europe and reimbursed the colonies for raising troops in North America. In July 1758, the British won their first great victory at Louisbourg, near the mouth of the St. Lawrence River. A month later, they took Fort Frontenac at the western end of the river. Then they closed in on Quebec, where Gen. James Wolfe won a spectacular victory on the Plains of Abraham, September 1759 (though both he and the French commander, the Marquis de Montcalm, were fatally wounded). With the fall of Montreal in September 1760, the French lost their last foothold in Canada. Soon, Spain joined France against England, and for the rest of the war Britain concentrated on seizing French and Spanish territories in other parts of the world.
Let's review. In the 1750's, English and French colonies were both expanding into the land west of the Appalachian Mountains, known as the Ohio Territory. When the French attacked a series of trading posts and then built a new fort in western Pennsylvania, the British government finally took action, sending George Washington to request that the French clear out. They refused. Unable to capture the French fort, Washington built his own, which the French army swiftly overtook. The French army and their Indian allies dominated the battlefields of the French and Indian War for three years until a change in British leadership, paired with an outbreak of smallpox among the Indians, gave the English the upper hand.