In January 1757 the Holy Roman Empire, led by Maria Theresa of Habsburg,
empress of Austria (although her husband Francis I was Holy Roman Emperor), declared war on Prussia, who now found herself surrounded by enemies, with much greater populations and resources. Frederick's response was to invade Bohemia, where he defeated the Austrians at the Battle of Prague (6 May 1757), although he was himself defeated at Kolin (18 June 1757) and forced to withdraw, although he then defeated a French and Austrian army in Saxony at the Battle of Rossbach (5 November 1757), and an Austrian army invading Silesia at the battle of Leuthen (5 December 1757. The same year saw Clive of India defeat the French in India at the Battle of Plassey (23 June), and the French occupy Hanover, having defeated the duke of Cumberland at Hastenbeck, forcing him to sign the Convention of Kloster-Zeven
The French occupation of Hanover was short lived, and a joint British and Hanoverian army defeated a Franco-Austrian one at Crefeld (June 1758), followed two months later by Frederick's victory over the Russians at Zorndorg (August 1758), halting their advance. The Austrians were able to inflict a rare defeat on Frederick at Hochkirck (October 1758), but failed to take advantage of it
1759 saw Prussia on the back foot, but Britain triumphant. Frederick was defeated by the Russians at Kunersdorf (August) and the Austrians at Maxen (November). In contrast, Britain was victorious on land, at sea, and in the colonies. August saw the Battle of Minden (1 August), where a combined British and Hanoverian force defeated a new French attack, and the naval Battle of Lagos (7-18 August 1759, off Portugal), where a French fleet intended for an invasion of England was defeated. September saw the capture of Quebec from the French, and 20 November the naval battle of Quiberon Bay (Brittany), the defeat of a French fleet intended for an invasion of Scotland.
British success continued in 1760, with victory over the French in India at the Battle of Wandiwash (Madras, 22 January), which ended French hopes of a victory in India. It also saw some success for Frederick II, despite a short occupation of Berlin by the Russians in October. He defeated the Austrians at Torgau (3 November), although losses were heavy on both sides. 1761 continued in the same vein, with the British successful at Pondicherry (January), and the Germans defeating the French at Villinghause (15 July). At this point, the nature of the war was changed by the death of two monarchs. First was the death of George II, and the accession of George III, who ended British aid to Prussia. Just when it looked like Prussia was doomed, Tsar Peter III succeeded to the Russian Throne (January 1762). The new Tsar was a great admirer of Frederick II, and quickly moved to end the war between Prussia and Russia (Treaty of St. Petersburg, 5 May 1762). The war now turned decisively towards Britain and Prussia. Frederick II defeated the Austrians at Burkersdorf (21 July 1762) and Reichenbach (16 August), regaining all of his lost territory, while the British captured Havana and Manila from the Spanish. Peace between Britain and France was restored by the Treaties of Fontainebleau (3 November 1763) and of Paris (10 February 1763), in which Britain restored Cuba and the Philippines to Spain, while retained her conquests from the French in Canada, America and India. Five days later the Treaty of Hubertusberg (15 February 1763) saw peace between Austrian, Prussia and Saxony, confirming Silesia as Prussian territory.
The Seven Years War saw Britain established as the greatest colonial power, with control over India and North America seemingly secured, while Prussia emerged as the greatest power on the Continent, and the dominant force inside Germany, reducing still further the power of the Holy Roman Empire and Habsburg Austria. Frederick II of Prussia (The Great) emerges as the most remarkable leader of the war. Prussia was the smallest of the main combatants, and yet Frederick survived year after year of campaigning, and despite coming near to defeat he emerged triumphant.