The Trafalgar campaign was a long and complicated series of fleet manoeuvres carried out by the combined French and Spanish fleets; and the opposing moves of the Royal Navy during much of 1805. These were the culmination of French plans to force a passage through the English Channel, and so achieve a successful invasion of the United Kingdom. The plans were extremely complicated and proved to be impractical. Much of the detail was due to the personal intervention of Napoleon, who as a soldier rather than a sailor failed to consider the effects of weather, difficulties in communication, and the Royal Navy. Despite limited successes in achieving some elements of the plan the French commanders were unable to follow the main objective through to execution. The campaign, which took place over thousands of miles of ocean, was marked by several naval engagements, most significantly at the Battle of Trafalgar on 21 October, where the combined fleet was decisively defeated, and from which the campaign takes its name. A final mopping up action at the Battle of Cape Ortegal on 4 November completed the destruction of the combined fleet, and secured the supremacy of the Royal Navy at sea.

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 southeast England. In 1796 the French had already tried to invade Ireland in order to destabilise the UK or as a stepping-stone to Great Britain. The first French Army of England had gathered on the Channel coast in 1798, but an invasion of England was sidelined by Napoleon's concentration on the campaigns in Egypt and against Austria, and shelved in 1802 by the Peace of Amiens. 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.