The sheer number of European wars is mind-boggling. Brecke has compiled a prequel to the Conflict Catalog which lists 1,148 conflicts from 900 CE to 1400 CE, and the catalog itself lists another 1,166 from 1400 CE to the present -- about two new conflicts a year for eleven hundred years. The vast majority of these conflicts, including most of the major wars involving great powers, are outside the consciousness of all but the most assiduous historians. To take some random examples, the Dano-Swedish War (1516-25), the Schmalkaldic War (1546-47), the Franco-Savoian War (1600-1601), the Turkish-Polish War (1673-76), the War of Julich Succession (1609-10), and the Austria-Sardinia War (1848-49) elicit blank stares from most educated people.

Warring was not just prevalent in practice but accepted in theory. Howard notes that among the ruling classes, "Peace was regarded as a brief interval between wars," and war was "an almost automatic activity, part of the natural order of things." Luard adds that while many battles in the 15th and 16th centuries had moderately low casualty rates, "even when casualties were high, there is little evidence that they weighed heavily with rulers or military commanders. They were seen, for the most part, as the inevitable price of war, which in itself was honourable and glorious."

The backdrop of European history during most of the past millennium is everpresent warring. Carried over from the knightly raiding and feuding in medieval times, the wars embroiled every kind of political unit that emerged in the ensuing centuries.

What were they fighting over? The motives were the "three principal causes of quarrel" identified by Hobbes: predation (primarily of land), preemption of predation by others, and credible deterrence or honor. The principal difference between European wars and the raiding and feuding of tribes, knights, and warlords was that the wars were carried out by organized political units rather than by individuals or clans. Conquest and plunder were the principal means of upward mobility in the centuries when wealth resided in land and resources rather than in commerce and innovation. Nowadays ruling a dominion doesn't strike most of us as an appealing career choice. But the expression "to live like a king" reminds us that centuries ago it was the main route to amenities like plentiful food, comfortable shelter, pretty objects, entertainment on demand, and children who survived their first year of life. The perennial nuisance of royal bastards also reminds us that a lively sex life was a perquisite of European kings no less than of harem-holding sultans, with "serving maids" a euphemism for concubines.

Though the motive to lead a dominant political bloc was constant through European history, the definition of the blocs changed, and with it the nature and extent of the fighting. In War in International Society, the most systematic attempt to combine a dataset of war with narrative history, Luard proposes that the sweep of armed conflict in Europe may be divided into five "ages," each defined by the nature of the blocs that fought for dominance. In fact Luard's ages are more like overlapping strands in a rope than boxcars on a track, but if we keep that in mind, his scheme helps to organize the major historical shifts in war.

But what the leaders sought was not just material rewards but a spiritual need for dominance, glory, and grandeur -- the bliss of contemplating a map and seeing more square inches tinted in the color that represents your dominion than someone else's. Luard notes that even when rulers had little genuine authority over their titular realms, they went to war for "the theoretical right of overlordship: who owed allegiance to whom and for which territories." Many of the wars were pissing contests. Nothing was at stake but the willingness of one leader to pay homage to another in the form of titles, courtesies, and seating arrangements. Wars could be triggered by symbolic affronts such as a refusal to dip a flag, to salute colors, to remove heraldic symbols from a coat of arms, or to follow protocols of ambassadorial precedence.

First, humans are altricial, with immature newborns and a long childhood. That means that a father can die while a son is too young to rule. Second, character traits are polygenic, and hence obey the statistical law called regression to the mean: however exceptional in courage or wisdom a parent may be, on average his or her children will be less so. (As the critic Rebecca West wrote, 645 years of the Habsburg dynasty produced "no genius, only two rulers of ability…, countless dullards, and not a few imbeciles and lunatics.") Third, humans reproduce sexually, which means that every person is the genetic legacy of two lineages, not one, each of which can lay a claim to the person's loyalties when he is alive and to his perquisites when he dies. Fourth, humans are sexually dimorphic, and though the female of the species may, on average, get less emotional gratification from conquest and tyranny than the male, many are capable of cultivating the taste when the opportunity presents itself. Fifth, humans are mildly polygynous, so males are apt to sire bastards, who become rivals to their legitimate heirs. Sixth, humans are multiparous, having several offspring over their reproductive careers. This sets the stage for parent-offspring conflict, in which a son may want to take over a lineage's reproductive franchise before a father is through with it; and sibling rivalry, in which a laterborn may covet the parental investment lavished on a firstborn. Seventh, humans are nepotistic, investing in their siblings' children as well as in their own. Each of these biological realities, and often several at a time, left room for disagreement about who was the appropriate successor of a dead monarch, and the Europeans hashed out these disagreements in countless dynastic wars.

Rulers always face the dilemma of how to reconcile their thirst for everlasting power with an awareness of their own mortality. A natural solution is to designate an offspring, usually a firstborn son, as a successor. Not only do people think of their genetic progeny as an extension of themselves, but filial affection ought to inhibit any inclination of the successor to hurry things along with a little regicide. This would solve the succession problem in a species in which an organism could bud off an adult clone of himself shortly before he died. But many aspects of the biology of Homo sapiens confound the scheme.

Luard calls the first of his ages, which ran from 1400 to 1559, the Age of Dynasties. In this epoch, royal "houses," or extended coalitions based on kinship, vied for control of European turfs. A little biology shows why the idea of basing leadership on inheritance is a recipe for endless wars of succession.

Luard designates 1559 as the inception of the Age of Religions, which lasted until the Treaty of Westphalia ended the Thirty Years' War in 1648. Rival religious coalitions, often aligning with rulers according to the principle Un roi, une loi, une foi (One king, one law, one faith), fought for control of cities and states in at least twenty-five international wars and twenty-six civil wars. Usually Protestants warred against Catholics, but during Russia's Time of Troubles (an interregnum between the reign of Boris Godunov and the establishment of the Romanov dynasty), Catholic and Orthodox factions vied for control. The religious fever was not confined to Christendom: Christian countries fought Muslim Turkey, and Sunni and Shiite Muslims fought in four wars between Turkey and Persia.

This is the age that contributed atrocities number 13, 14, and 17 to the population-adjusted top-twenty-one list on page 195, and it is marked by pinnacles of death in figure 5-15 and figure 5-18. The era broke new records for killing partly because of advances in military technology such as muskets, pikes, and artillery. But that could not have been the main cause of the carnage, because in subsequent centuries the technology kept getting deadlier while the death toll came back to earth. Luard singles out religious passion as the cause:

It was above all the extension of warfare to civilians, who (especially if they worshipped the wrong god) were frequently regarded as expendable, which now increased the brutality of war and the level of casualties. Appalling bloodshed could be attributed to divine wrath. The duke of Alva had the entire male population of Naarden killed after its capture (1572), regarding this as a judgement of God for their hard-necked obstinacy in resisting; just as Cromwell later, having allowed his troops to sack Drogheda with appalling bloodshed (1649), declared that this was a "righteous judgement of God." Thus by a cruel paradox those who fought in the name of their faith were often less likely than any to show humanity to their opponents in war. And this was reflected in the appalling loss of life, from starvation and the destruction of crops as much as from warfare, which occurred in the areas most ravaged by religious conflict in this age.

Names like the "Thirty Years' War" and the "Eighty Years' War," together with the never-equaled spike in war durations shown in figure 5-14, tell us that the Wars of Religion were not just intense but interminable. The historian of diplomacy Garrett Mattingly notes that in this period a major mechanism for ending war was disabled: "As religious issues came to dominate political ones, any negotiations with the enemies of one state looked more and more like heresy and treason. The questions which divided Catholics from Protestants had ceased to be negotiable. Consequently… diplomatic contacts diminished." It would not be the last time ideological fervor would act as an accelerant to a military conflagration.