On June 18, 1815, near a small Belgian village, the fate of Europe was decided in a single day. Napoleon Bonaparte, returned from exile and commanding a reconstituted French army, faced a coalition of British, Dutch, and Prussian forces under Wellington and Blücher. By nightfall, the French army had collapsed, Napoleon's political survival had ended, and the continent's borders were about to be redrawn by diplomats in Vienna who had spent months constructing a new European order on the assumption that this moment would come. This book reconstructs Waterloo through regimental after-action reports, officer correspondence, soldier memoirs, Prussian and British staff records, and contemporary cartographic evidence. It examines the tactical decisions of June 18 in granular detail — the delayed French opening, the failure at Hougoumont, the timing of the Prussian arrival — while situating the battle within the broader strategic and political context of the Hundred Days campaign that preceded it. The narrative moves beyond the battlefield to examine what Waterloo settled and what it did not: the Congress of Vienna's construction of a conservative European order, the myth-making that immediately surrounded the battle on all sides, and the long nineteenth century of relative great-power peace that the 1815 settlement made possible. It also examines Napoleon himself in these final weeks — the strategic instincts still present, the institutional support irreparably eroded. A rigorously sourced account of a single day whose consequences reordered a continent and whose historical memory has never stopped being contested.