Carl von Clausewitz’s theories of war have influenced generations of military leaders and policy makers throughout the Western world. On War is widely regarded as Clausewitz’s premier work on the philosophy of war and the first modern book of military strategy. The work is controversial—Clausewitz’s argument that war cannot be waged in a limited way has been blamed for the massive casualties in World Wars I and II—yet it remains required reading at many military academies. Its enduring relevance can be explained by Clausewitz's recognition that while political leaders are not military experts, “a certain grasp of military affairs is vital for those in charge of general policy.” He also emphasized social and economic issues as some of the most important factors in deciding the outcomes of wars. Clausewitz described a “remarkable trinity,” consisting of “the people, the commanders and their army, and the government.” The interplay among these three parts determines the nature, conduct, and purpose of war. More than a century after they first appeared in print, Clausewitz’s theories continue to be debated and discussed among people living in a world very different from his own. In our modern age of nuclear threats and terrorism committed by groups with no national ties, On War remains essential reading.