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Beginning of the Seven Years' War

The peace achieved by Louis with the Treaty of Aix-la-Chapelle lasted only seven years. At the end of August 1755, Marie Therese, the Empress of Austria, discreetly wrote a letter to Louis XV, which was passed by the Austrian ambassador in Paris to Madame de Pompadour for delivery to the King. She proposed a secret alliance between Austria and France, to meet the threats of the growing power of Prussia, which was still formally an ally of France, and Britain.

In the New World, conflict had already begun between Great Britain and France. New France was at an enormous demographic disadvantage vis-à-vis its British counterparts; there were about 70,000 French colonists spread over a territory from the St. Lawrence River to the Great Lakes and extending down the Ohio and Mississippi River valleys to Louisiana, compared with a population of one million settlers in the British 13 Colonies. To defend its territories, France had constructed Fort Duquesne to defend its frontier against the indigenous Americans; Britain sent the young George Washington with a small force to construct his own fortification, Fort Necessity, nearby. In 1754, after the killing of French envoy, Joseph Coulon de Jumonville, the French sent reinforcements and compelled Washington and his men to withdraw.

The undeclared French and Indian War followed, with British and French colonies in North America engaging in open conflict. By the end of 1755, British ships had captured over 300 French merchantmen. In January 1756, Louis sent an ultimatum to London, which was rejected by the British government. A few months later, on 16 January 1756, Frederick the Great of Prussia signed the Treaty of Westminster, allying himself with Britain. Louis responded immediately on 1 May 1756 by sealing a formal defensive treaty with Austria, the first Treaty of Versailles, offering to defend Austria in case of a Prussian attack. This was a complete reversal of France's historic conflict with Austria, which had been underway for nearly two hundred years, and it was shocking to many in the French Court.

Louis declared war on Great Britain on 9 June 1756, confident of success. The French navy quickly defeated a British fleet in the Mediterranean and captured Minorca from Britain. Meanwhile, the French army greatly outnumbered its British and Prussian counterparts on the continent, and after some fighting France signed the Convention of Klosterzeven with the Duke of Cumberland, which resulted in French troops occupying parts of Hanover and the Hanoverians withdrawing from the conflict entirely. Another French army had already invaded Saxony and Hanover, the ancestral home of George II. However, the best French commander, Maurice de Saxe, had died two years after the War of the Austrian Succession, and his successors, the Prince de Soubise, the Duke D'Estrees and the Duke de Broglie detested each other, and were rarely willing to cooperate.

In August Frederick of Prussia made a lightning strike into Saxony and on 5 November 1757, though outnumbered by the French nearly two to one, decisively defeated the army of the Prince de Soubise at the Battle of Rossbach. The new British Prime Minister, William Pitt, named a new commander, Duke Ferdinand of Brunswick-Wolfenbüttel, and the French armies were gradually pushed back to the Rhine, and defeated again at the Battle of Krefeld on 23 June. Thereafter, Britain and Prussia held the upper hand, tying down the French army in the German states along the Rhine.

British naval supremacy prevented France from reinforcing its colonies overseas, and British naval squadrons raided the French coast at Cancale and Le Havre and landed on the Ile d'Aix and Le Havre. In 1759 the British attacked Martinique and Guadeloupe in the French West Indies, and captured Port Louis and Quebec. A series of French naval defeats forced Louis to abandon plans for invasion of Britain. In India, the French colony at Pondicherry was besieged by the British, and surrendered the following year. On 8 September 1760, Montreal surrendered, bringing to an end French rule in Canada. Martinique fell to the British in 1762.