By the early ninth century, the Carolingian empire was at the height of its power. The Franks, led by Charlemagne, had built the largest European domain since Rome in its heyday. Though they jockeyed for power, prestige, and profit, the Frankish elites enjoyed political and cultural consensus. But just two generations later, their world was in shambles. Civil war, once an unthinkable threat, had erupted after Louis the Pious’s sons overthrew him—and then placed their knives at the other’s neck. Families who had once charged into battle together now drew each other’s blood. The Carolingian Civil War would rage for years as kings fought kings, brother faced off against brother, and sons challenged fathers. Oathbreakers is the dramatic history of this brutal, turbulent time. Medieval historians David M. Perry and Matthew Gabriele illuminate what happens when a once unshakeable political and cultural order breaks down and long suppressed tensions flare into deadly violence. Drawn from rich primary sources, featuring a wide cast of characters, packed with dramatic twists and turns, this is history that rivals the greatest fictional epics—with consequences that continue to shape our own world.