Allemand's expedition of 1805, often referred to as the Escadre invisible in French sources, was an important French naval expedition during the Napoleonic Wars, which formed a major diversion to the ongoing Trafalgar Campaign in the Atlantic Ocean. With the French Mediterranean Fleet at sea, Emperor Napoleon I hoped to unite it with the French Atlantic Fleet and together form a force powerful enough to temporarily displace the British Royal Navy Channel Fleet for long enough to allow an invasion force to cross the English Channel and land in Britain. In support of this plan, the French squadron based at Rochefort put to sea in July 1805, initially with the intention that they would join the Atlantic Fleet from Brest. When this fleet failed to put to sea, the Rochefort squadron, under Contre-Admiral Zacharie Allemand, went on an extended raiding cruise across the Atlantic, both to intercept British trade left lightly defended by the concentration of British forces in European waters and with the intention of eventually combining with the French Mediterranean Fleet then blockaded in Spanish harbours.

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.