There are two more distortions, however, that need to be corrected. First is the idea that intolerance reigned in the Middle Ages, as everybody in Europe ran around burning people for disputing the finer points of Christian doctrine, until the benevolent secular state arrived in the Early Modern era and set us straight. And second is the even more intractable notion that religious people were fighting all the time, until the peace-loving nation state curbed the power of lunatic religious fanatics and put a stop to all that Crusading, witch-burning, Inquisiting, Warring of Religions, and so on. Both of these are, to say it no more strongly, a long way off the mark.
 
Start with witch-hunting:

As entertaining as it might be, for instance, to think of the Middle Ages as a time of inquisitors burning thousands of witches at the stake, it was not until the early modern period – especially from the late sixteenth century through the middle of the seventeenth – that a great enthusiasm for hunting out and prosecuting witches sprang up in various regions of Western Europe … As far as the church’s various regional inquisitions are concerned, their principal role in the early modern witch hunts was to suppress them: to quiet mass hysteria through the imposition of judicial process, to restrain the cruelty of secular courts, and to secure dismissals in practically every case.

Hart cites a host of Christian bishops, leaders and even popes who bear witness to this point, and then turns his attention to the people who actually conducted the witch-hunting. Perhaps surprisingly to us, it was primarily believers in the sovereignty of the secular state who favoured the punishment of alleged witches, and primarily Christians who opposed it:

Thomas Hobbes (1588-1679), for instance, the greatest modern theorist of complete state sovereignty, thought all religious doctrine basically mendacious and did not really believe in magic; but still he thought witches should continue to be punished for the good of society. The author of De la démonomanie des sorciers (1580), perhaps the most influential and (quite literally) inflammatory of all the witch-hunting manifestos of its time, was Jean Bodin (c.1530-1596), who believed witches should be burned at the stake … But Bodin was also the first great theorist of that most modern of political ideas, the absolute sovereignty of the secular state.

In contrast, Rodney Stark writes, ‘The first significant objections to the reality of satanic witchcraft came from Spanish inquisitors, not from scientists.’
 
Even the Spanish Inquisition, the details of which are only known to most of us via Monty Python, was both far more opposed by the church, and far more conducted under the auspices of the state – partly to enforce national unity following the reconquista, partly to ensure limpieza de sangre (racial purity) – than often realised. Far from being proof of the inherent intolerance of true Christianity, Hart argues,

… it seems obvious that the true lesson to be learned is just the opposite: the inherent violence of the state, and the tragedy that the institutional church was ever assumed into temporal politics, or ever became responsible for the maintenance of social order or of national or imperial unity … During the so-called Dark Ages, in fact, the only penalty for obdurate heresy was excommunication … Violence increased in proportion to the degree of sovereignty claimed by the state.

Perhaps unsurprisingly, Hart applies the same logic to the violence of early modernity. In the medieval period, there was a widespread concept of overlapping spheres of authority, loyalty and mandate, such that the prince had authority over some aspects of life and the church over others. In the early modern period, however, this idea gave way to the absolute power of the monarch, in whom both the power of the state and the divine right to govern, even over the church if necessary, existed simultaneously. In Western Europe, the so-called ‘wars of religion’ were, in the main, struggles for power between noble families – Habsburgs, Valoises, Guises, Bourbons, Montmorencies and so on – and were political and ideological rather than essentially religious conflicts (most of the fighting happened between Catholics anyway). The Thirty Years’ War, which again was between two Catholic houses, Bourbon and Habsburg, was once more a struggle between those who wanted state absolutism and those who wanted the old imperial system, and should therefore not be called a ‘war of religion’ at all:

the mercenary armies whose predatory brutality to the towns and villages of the German states was part of the special horror of the Thirty Years’ War were scarcely motivated by disputes over papal primacy or transubstantiation.

Moreover, the Peace of Westphalia, which brought the Thirty Years’ War to an end, in beginning the era of the nation state, may ultimately have been responsible for more bloodshed than almost any other treaty in history. Far from finally setting Europe’s religious squabbles aside and ushering in an era of peace, Westphalia represented the defeat of imperialism (in which the church retained a moral authority over all of Christendom, which it frequently used to preserve peace and prevent aggression), and the victory of nationalism (in which national interests were not subject to any external constraint, other than military force). The consequences were grave for Europe as a whole:

The slow, convulsive, miserable, violent death of the Holy Roman Empire, both before and after Westphalia, belonged to the first phase of a new age of territorial and (ultimately) ideological wars, nationalist and (then) imperialist wars, wars prompted by commerce, politics, colonial interests, blood and soil, and (at the last) visions of the future of Europe and even of humanity: England’s wars with the Netherlands, Spain, Portugal and France, Sweden’s wars with Poland, Russia and Denmark, France’s wars with Spain, the Netherlands, and the League of Augsburg; the war of the Spanish succession, the war of the Polish succession, the two Silesian wars of Austrian succession, the third Silesian war; revolutionary France’s wars with Britain, Holland and Spain, the wars of the First, Second, and Third Coalitions, and all the Napoleonic wars; the wars of Italian unification, the wars of German unification, the Franco-Prussian War; the first and second Balkan wars, the First World War, the Second World War … (to name just the most obvious examples).

People kill for all sorts of reasons, of course. But it is hard to argue, on the basis of the number of people killed, that the rise of the secular nation state brought peace, in contrast to an epoch of medieval war-mongering. The facts suggest just the opposite. And whereas the Thirty Years’ War was ‘a scandal to the consciences of the nations of Europe’, by the mid-twentieth century, morality in these secular states had become so tainted, and violence so common, that ‘liberal democracies did not scruple to bomb open cities from the air, or to use incendiary or nuclear devices to incinerate tens of thousands of civilians, sometimes for only the vaguest of military objectives.’ Westphalia, for Hart, was not the end of the age of needless European wars, but the beginning.
 
Which means, I guess, that people who say that religion is the cause of all wars simply have no idea what they are talking about.