The Battle of Trafalgar, fought on 21 October 1805, was preceded by one of the largest-scale campaigns in the Age of Sail. Involving the fleets of Britain, France and Spain, numbering over a hundred battleships, and many more smaller vessels, together with the armies of France, Britain and Austria, it lasted over ten months and covered a vast area bounded by the Channel in the north and the Mediterranean in the south – and at one point extending even to the West Indies.

Essentially, the issue at stake was the future of Great Britain. Napoleon, by then Emperor of the French, having crowned himself in Notre Dame Cathedral in Paris on 4 December 1804, was determined to eliminate his most persistent opponent. He took advantage of the quiescence of all the other major European powers to mass an army on the French Channel coast, where, throughout 1804, with his customary energy and administrative ability, he supervised the construction of a large flotilla of invasion barges.

The British responded to the threat by organising a three-tiered defence. First there were battlefleets, holding the main French fleets in check in Brest, Rochefort and Toulon. Then there were the squadrons of smaller vessels stationed in the Channel itself under Admiral Lord Keith, ready to harry the French invasion forces as soon as they emerged from their ports, Finally, ashore, there were land defences manned by a mixture of regulars and specially raised militia and volunteers.

Napoleon realised that to protect his vulnerable troop carriers as they made their hazardous crossing, he had to bring a large naval force into the Channel. To achieve this, in late 1804 he devised an ambitious plan, designed to draw the British fleets from their usual stations. He ordered his admirals to emerge from port and rendezvous in the West Indies, where they would attack the rich British possessions, thus forcing the Royal Navy to divert ships for their protection. The French were then to return swiftly to the Channel in a concentrated force, large enough to sweep aside the British defenders and take up a position to cover the invasion flotilla.