By mid-1742, the War of the Austrian Succession had broken out in Europe. Principally fought by Prussia and Austria over possession of Silesia, the war soon engulfed most of the major powers of Europe, who joined two competing alliances. The scale of this new war dwarfed any of the fighting in the Americas, and drew Britain and Spain's attention back to operations on the European continent. The return of Vernon's fleet in 1742 marked the end of major offensive operations in the War of Jenkins' Ear. France entered the war in 1744, emphasizing the European theatre and planning an ambitious invasion of Britain. While it ultimately failed, the threat persuaded British policymakers of the dangers of sending significant forces to the Americas which might be needed at home. Britain did not attempt any additional attacks on Spanish possessions. In 1745, William Pepperrell of New England led a colonial expedition, supported by a British fleet under Commodore Peter Warren, against the French fortress of Louisbourg on Cape Breton Island off Canada. Pepperrell was knighted for his achievement, but Britain returned Louisbourg to the French by the Treaty of Aix-La-Chapelle in 1748. A decade later, during the Seven Years' War , British forces under Lord Jeffrey Amherst and General Wolfe recaptured it.. How many years after William Pepperrell took the French fortress of Louisbourg in 1745 did Britain have to return Louisbourg back to the French by the Treaty of Aix-La-Chapelle?