World War I not only toppled empires and redrew borders—it remade the modern world, with few nations feeling the effects as profoundly as Great Britain. The country’s deadliest war—from which 6 percent of its men never returned—accelerated sweeping social, economic and political changes that fractured the rigid British class system and weakened the aristocracy. For centuries, hereditary landowners exercised a near feudal dominance of the British countryside. As late as 1873, fewer than 5 percent of Britons owned all of England’s property. But an agricultural depression in the late 1800s had already begun to erode their dominance as the First...