Clash of Fleets is an operational history that records every naval engagement fought between major surface warships during World War I. Much more than a catalog of combat facts, Clash of Fleets explores why battles occurred; how the different navies fought; and how combat advanced doctrine and affected the development and application of technology. The result is a holistic overview of the war at sea as it affected all nations and all theaters of war. A work of this scope is unprecedented. Organized into seven chapters, the authors first introduce the technology, weapons, ships, and the doctrine that governed naval warfare in 1914. The next five chapters explore each year of the war and are subdivided into sections corresponding to major geographic areas. This arrangement allows the massive sweep of action to be presented in a structured and easy to follow format that includes engagements fought by the Austro-Hungarian, British, French, German, Ottoman, and Russian Navies in the Adriatic, Aegean, Baltic, Black, Mediterranean, and North Seas as well as the Atlantic, India, and Pacific Oceans. The role of surface combat in the Great War is analyzed and these actions are compared to major naval wars before and after. In addition to providing detailed descriptions of actions in their historical perspectives, O’Hara and Heinz advance several themes, including the notion that World War I was a war of navies as much as a war of armies. They explain that surface combat had a major impact on all aspects of the naval war and on the course of the war in general. Finally, Clash of Fleets illustrates that systems developed in peace do not always work as expected in war, that some are not used as anticipated, and that others became unexpectedly important. There is much for today’s naval professional to consider in the naval conflict that occurred a century ago.