Yale's Nota Bene imprint has brought out a second edition of Jonathan Riley-Smith's The Crusades: A History ($20), which pulls off the enviable feat of summing up seven centuries of religious warfare in a crisp 309 pages of text. Riley-Smith helps account for the Crusades' long run by observing the canny methods of recruitment. First, a crusade was billed as a war not of aggression but of righteousness -- "fought against those perceived to be the external or internal foes of Christendom for the recovery of Christian property or in defence of the Church or Christian people." Second, crusaders weren't asked to give their time and lives to a cause so much as to do themselves a spiritual favor. Joining up was a penance, "that is, an act, often of self-punishment, which constituted an attempt to repay the debt owed to God on account of sin." The crusader's reward was an indulgence, a cancellation of punishment due for previous sins. And so "a crusade was for the crusader as an individual only secondarily about service-in-arms to God or the benefiting of the Church or Christianity; it was primarily about benefiting himself since he was engaged in an act of self-sanctification."

Not quite as short but still impressively concise is a classic work by an author whom Anthony Grafton calls in his introduction "the greatest narrative historian of her century." That would be C.V. (for Cicely Veronica) Wedgwood, a descendant of the pottery family, and her work is The Thirty Years War (NYRB, $18.95), originally published in 1938. It took Wedgwood just over 500 pages to bring the war from its beginning, with the tragicomic defenestration of Prague in 1618, to its end, with the singing of the "Te Deum" and the clanging of church bells in the same city, in 1648. Before launching into the war proper, however, Wedgwood filled in the background of pervasive Middle European religious intolerance that made the fighting so difficult to control: "The energy of the educated was perverted into the writing of scurrilous books, which were joyfully received by an undiscriminating public. The Calvinists exhorted all true believers to violence and took special delight in the more bloodthirsty psalms. But the Catholics and Lutherans were not innocent and force was everywhere the proof of true faith. . . . Calvinist services in Styria [now part of Austria] were frequently interrupted by Jesuits disguised among the congregation who would tweak the prayer book from the hands of the worshipper and deftly substitute a breviary."

In The Conquest of Morocco (Farrar Straus Giroux, $15), Douglas Porch, a frequent contributor to Book World, covers a different form of warfare: the colonial variety. Lacking either the long imperial tradition of her rival Great Britain or strong economic motives, France had to find a way of justifying her desire to subjugate Morocco, a process completed in the decade before World War I. The leader of the French forces, Gen. Louis Lyautey, was up to the task: "Only by claiming that he was 'civilizing' Morocco, that the Moroccans actually preferred the French presence to their normal state of anarchy, could he sell colonial expansion to a French public skeptical of its value. . . . [French officers'] novel methods of colonial warfare, which combined economic and military penetration, persuasion with force, were developed largely as a public-relations exercise to convince France itself that colonialism benefited the conquered."

The Great War, as World War I was called before anyone knew there would be a II, was, among other things, a horrific learning experience. Optimists learned that humankind could revert to a state of "the primitive, an atavistic upsurge of inter-ethnic violence." Commanders found new weapons to deploy. And everyone discovered that trench warfare was a monstrous way to ensure military stalemate -- as well as to waste millions of lives. David Stevenson addresses these features of the war in Cataclysm (Basic, $18.95). But as suggested by his subtitle, "The First World War as Political Tragedy," he is most concerned with what he considers the calculated risk-taking that shaped the conflict. Arguing against those who attribute the onset of war to a fateful series of accidents, Stevenson emphasizes in his introduction that going to war is always "a political act, a product both of supercharged emotions and of reason and will."

-- Dennis Drabelle