The Battle of Trafalgar (21 October 1805) was a sea battle fought between the British Royal Navy and the combined fleets of the French Navy and Spanish Navy. It was the most decisive battle of the First Great European War, where the French-Spanish fleet was able to defeat the Royal Navy fleet commanded by Lord Nelson, who was killed in the battle, and opened up the way to allow Emperor Napoleon I to invade Great Britain. Although Napoleon did not actually invade, the threat was enough to force war-weary Britain to agree to the Treaty of Copenhagen