Napoleon's planned invasion of the United Kingdom at the start of the War of the Third Coalition, although never carried out, was a major influence on British naval strategy and the fortification of the coast of South East England.

Building on planning for mooted invasions under France's ancien régime in 1744, 1759, and 1779, preparations began again in earnest soon after the outbreak of war in 1803, and were finally called off in 1805, before the Battle of Trafalgar.

A large "National Flotilla"[1] of invasion barges was built in Channel ports along the coasts of France and the Netherlands (then under French domination as the Batavian Republic), all the way from Étaples to Flushing, and gathered at Boulogne.

Port facilities at Boulogne were improved (even though its tides made it unsuitable for such a role) and forts built, whilst the discontent and boredom that often threatened to overflow among the waiting troops was allayed by constant training and frequent ceremonial visits by Napoleon himself (including the first ever awards of the Légion d'honneur).

[6][7] For his planned subsidiary invasion of Ireland, Napoleon had formed an Irish Legion in 1803, to create an indigenous part of his 20,000-man Corps d'Irelande.

This plan was typical of Napoleon in its dash and reliance on fast movement and surprise, but such a style was more suited to land than to sea warfare, with the vagaries of tide and wind and the effective British blockade making it ever more impractical and unlikely to succeed as time passed.

Therefore, on 27 August 1805, Napoleon used the invasion army as the core of the new Grande Armée and had it break camp and march eastwards to begin the Ulm campaign.