answers.input_text: short
answers.span_text: Napoleon 's planned invasion of the United Kingdom
questions: why did britain declare war on france in 1803
name: British anti-invasion preparations of 1803–05
contexts: 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 .