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British motivations

Britain ended the uneasy truce created by the Treaty of Amiens when it had declared war on France in May 1803. The British were increasingly angered by Napoleon's reordering of the international system in Western Europe, especially in Switzerland, Germany, Italy and the Netherlands. Kagan argues that Britain was especially alarmed by Napoleon's assertion of control over Switzerland. The British felt insulted when Napoleon said it deserved no voice in European affairs (even though King George was an elector of the Holy Roman Empire) and sought to restrict the London newspapers that were vilifying him.

Britain had a sense of loss of control, as well as loss of markets, and was worried by Napoleon's possible threat to its overseas colonies. McLynn argues that Britain went to war in 1803 out of a "mixture of economic motives and national neuroses—an irrational anxiety about Napoleon's motives and intentions." McLynn concludes that it proved to be the right choice for Britain because, in the long run, Napoleon's intentions were hostile to the British national interest. Napoleon was not ready for war, and so this was the best time for Britain to stop them. Britain seized upon the Malta issue, refusing to follow the terms of the Treaty of Amiens and evacuate the island.

The deeper British grievance was their perception that Napoleon was taking personal control of Europe, making the international system unstable, and forcing Britain to the sidelines. Numerous scholars have argued that Napoleon's aggressive posture made him enemies and cost him potential allies. As late as 1808, the continental powers affirmed most of his gains and titles, but the continuing conflict with Britain led him to start the Peninsular War and the invasion of Russia, which many scholars see as a dramatic miscalculation.

There was one serious attempt to negotiate peace with France during the war, made by Charles James Fox in 1806. The British wanted to retain their overseas conquests and have Hanover restored to George III in exchange for accepting French conquests on the continent. The French were willing to cede Malta, Cape Colony, Tobago, and French Indian posts to Britain but wanted to obtain Sicily in exchange for the restoration of Hanover, a condition which the British refused.

Unlike its many coalition partners, Britain remained at war during the period of the Napoleonic Wars. Protected by naval supremacy (in the words of Admiral Jervis to the House of Lords "I do not say, my Lords, that the French will not come. I say only they will not come by sea"), Britain did not have to spend the entire war defending itself and could thus focus on supporting its embattled allies, maintaining low-intensity land warfare on a global scale for over a decade. The British government paid out a large amount of money to other European states so that they could pay armies in the field against France. These payments are colloquially known as the Golden Cavalry of St George. The British Army provided long-term support to the Spanish rebellion in the Peninsular War of 1808–1814, assisted by Spanish guerrilla ('little war') tactics. Anglo-Portuguese forces under Arthur Wellesley supported the Spanish, who campaigned successfully against the French armies, eventually driving them from Spain and allowing Britain to invade southern France. By 1815, the British Army played the central role in the final defeat of Napoleon at Waterloo.

Beyond minor naval actions against British imperial interests, the Napoleonic Wars were much less global in their scope than preceding conflicts such as the Seven Years' War, which historians term a " world war ".