British anti-invasion preparations of 1803–05

British anti-invasion preparations of 1803–05 were the military and civilian responses in the United Kingdom to Napoleon's planned invasion of the United Kingdom. They included mobilization of the population on a scale not previously attempted in Britain, with a combined military force of over 615,000 in December 1803. Much of the southern English coast was fortified to repel a French landing. However Napoleon never attempted his planned invasion and so the preparations were never put to the test.

In the aftermath of the French Revolution of 1789 Britain and France were at war almost continuously from 1793 to 1802, and then from 1803 to 1815, interrupted by the brief Peace of Amiens of 1802–3 and Napoleon's first exile at Elba in 1814–15. In late 1797, Napoleon said to the French Directory that:

[France] must destroy the English monarchy, or expect itself to be destroyed by these intriguing and enterprising islanders...Let us concentrate all our efforts on the navy and annihilate England. That done, Europe is at our feet.

However Napoleon decided against invading for the time being and instead unsuccessfully attacked British interests in Egypt. In March 1802 the two countries signed the Treaty of Amiens, which brought to an end nearly nine years of war. However both the British Prime Minister Henry Addington and Napoleon viewed the peace as temporary, and so it was, with Britain declaring war on France on 18 May 1803.William Pitt replaced Addington as Prime Minister on 10 May 1804.

In 1803 Napoleon turned his attention to invading England once more, saying: "All my thoughts are directed towards England. I want only for a favourable wind to plant the Imperial Eagle on the Tower of London". Napoleon now planned an invasion on a bigger scale than 1798 and 1801, and built a new armada for the effort. He assembled the Grande Armée of over 100,000 at Boulogne.

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