Hostility between France and Britain, particularly over the colonization of North America, continued. Fearing a French invasion of Hanover, George aligned himself with Prussia (ruled by his nephew, Frederick the Great), Austria's enemy. Russia and France allied with Austria, their former enemy. A French invasion of the British-held island of Minorca led to the outbreak of the Seven Years' War in 1756. Public disquiet over British failures at the start of the conflict led to Newcastle's resignation and the appointment of William Cavendish, 4th Duke of Devonshire, as prime minister and William Pitt the Elder as Secretary of State for the Southern Department. In April the following year George dismissed Pitt in an attempt to construct an administration more to his liking. Over the succeeding three months attempts to form another stable ministerial combination failed. In June Lord Waldegrave held the seals of office for only four days. By the start of July Pitt was recalled, and Newcastle returned as prime minister. As Secretary of State, Pitt guided policy relating to the war. Great Britain, Hanover, and Prussia and their allies Hesse-Kassel and Brunswick-Wolfenbüttel fought against other European powers, including France, Austria, Russia, Sweden and Saxony. The war involved multiple theatres from Europe to North America and India, where British dominance increased with the victories of Robert Clive over French forces and their allies at the Battle of Arcot and the Battle of Plassey.