This article argues that historians have overlooked and underplayed a major British strategic error, namely that Napoleon’s escape from Elba in February 1815 was a preventable calamity that put the hard-won victory of the sixth coalition at risk. It had the potential to change the course of history because the allied victory at Waterloo was by no means assured. All of the allies bear some portion of the blame for Napoleon’s escape, but none more so than the British, and especially the Royal Navy, which was the only allied service capable of preventing Napoleon’s escape. Instead, Lord Castlereagh prioritized coalition politics over Napoleon’s fate, and the navy prioritized the ongoing War of 1812 and demobilization over Mediterranean security. The British committed these blunders even though they knew that Napoleon was a flight risk, and even though the cost of reinforcing Napoleon’s guard was dwarfed by the cost of dealing with his escape.