In the Great War of 1914–18, a conflict distinctive first and foremost for its unprecedented bloodshed, less than 1 percent of the 8.5 million combatant deaths were naval personnel lost at sea. Such a disproportionate distribution of the human sacrifice might lead one to conclude that the Great War at sea had, at best, a peripheral significance to the final outcome, and yet no serious scholar has ever made such an argument. Prior to 1914, in history’s most expensive arms race to date, Britain defied the expectations of Germany in making the financial sacrifice necessary to maintain its naval superiority. Largely because of this superiority, the Allies were able to keep the fleets of the Central Powers contained in the North Sea, Baltic, and Adriatic, and to impose blockades on Germany and Austria-Hungary that, by 1916, contributed to serious food shortages in both countries. Faced with an insurmountable Allied supremacy in surface warships, the Central Powers attempted to revolutionize naval warfare by giving a central, offensive role to the submarine, a vessel originally conceived for a peripheral, defensive role (primarily as a harbor defender, against enemy blockade). In refocusing their efforts on undersea warfare, they created the issues that prompted the United States to intervene in a war in Europe, an unprecedented and, ultimately, decisive development. The focus on submarine warfare also caused the Central Powers to leave their capital ships rusting at anchor for much of the war, with dire consequences for the morale of most of their seamen. In 1917–18, Germany and Austria-Hungary (along with Russia, whose Baltic and Black Sea fleets had been similarly idled) experienced serious naval mutinies, and revolutionary movements in all three countries attracted significant numbers of sailors. By the end of the war, the victory of the Allies against the submarine challenge, following on their earlier success in sweeping the seas of German cruisers and other surface raiders, left them free to use the world’s sea lanes to transport supplies and troops to Europe from their overseas territories, and eventually from the United States, without which their ultimate victory could not have been accomplished. Thus, while the overwhelming majority of the effort, and the casualties, came on land, the action at sea was undeniably decisive to the outcome of the war.