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Seven Years' War

Frederick of Prussia's invasion of Saxony in August 1756 began a Third Silesian War and sparked the wider Seven Years' War. Maria Theresa and Prince Kaunitz wished to exit the war with possession of Silesia. Before the war started, Kaunitz had been sent as an ambassador to Versailles from 1750 to 1753 to win over the French. Meanwhile, the British rebuffed requests from Maria Theresa to aid her in reclaiming Silesia, and Frederick II himself managed to secure the Treaty of Westminster (1756) with them. Subsequently, Maria Theresa sent Georg Adam, Prince of Starhemberg, to negotiate an agreement with France, and the result was the First Treaty of Versailles of 1 May 1756. Thus, the efforts of Kaunitz and Starhemberg managed to pave a way for a Diplomatic Revolution; previously, France was one of Austria's archenemies together with Russia and the Ottoman Empire, but after the agreement, they were united by a common cause against Prussia. However, historians have blamed this treaty for France's devastating defeats in the war, since Louis XV was required to deploy troops in Germany and to provide subsidies of 25–30 million pounds a year to Maria Theresa that were vital for the Austrian war effort in Bohemia and Silesia.

On 1 May 1757, the Second Treaty of Versailles was signed, whereby Louis XV promised to provide Austria with 130,000 men in addition to 12 million florins yearly. They would also continue the war in Continental Europe until Prussia could be compelled to abandon Silesia and Glatz. In return, Austria would cede several towns in the Austrian Netherlands to the son-in-law of Louis XV, Philip of Parma, who in turn would grant his Italian duchies to Maria Theresa.

Maximilian von Browne commanded the Austrian troops. Following the indecisive Battle of Lobositz in 1756, he was replaced by Prince Charles Alexander of Lorraine, Maria Theresa's brother-in-law. However, he was appointed only because of his familial relations; he turned out to be an incompetent military leader, and he was replaced by Leopold Joseph von Daun, Franz Moritz von Lacy and Ernst Gideon von Laudon. Frederick himself was startled by Lobositz; he eventually re-grouped for another attack in June 1757. The Battle of Kolín that followed was a decisive victory for Austria. Frederick lost one third of his troops, and before the battle was over, he had left the scene. Subsequently, Prussia was defeated at Hochkirch in Saxony on 14 October 1758, at Kunersdorf in Brandenburg on 12 August 1759, and at Landeshut near Glatz in June 1760. Hungarian and Croat light hussars led by Count Hadik raided Berlin in 1757. Austrian and Russian troops even occupied Berlin for several days in August 1760. However, these victories did not enable the Habsburgs to win the war, as the French and Habsburg armies were destroyed by Frederick at Rossbach in 1757. After the defeat in Torgau on 3 November 1760, Maria Theresa realised that she could no longer reclaim Silesia without Russian support, which vanished after the death of Empress Elizabeth in early 1762. In the meantime, France was losing badly in America and India, and thus they had reduced their subsidies by 50%. Since 1761, Kaunitz had tried to organise a diplomatic congress to take advantage of the accession of George III of Great Britain, as he did not really care about Germany. Finally, the war was concluded by the Treaty of Hubertusburg and Paris in 1763. Austria had to leave the Prussian territories that were occupied. Although Silesia remained under the control of Prussia, a new balance of power was created in Europe, and Austrian position was strengthened by it thanks to its alliance with the Bourbons in Madrid, Parma and Naples. Maria Theresa herself decided to focus on domestic reforms and refrain from undertaking any further military operations.