Ending a war is much more difficult than starting one. However »inevitable« a war may look in hindsight, short-term causes matter at least as much as – and often more than – long-term tensions. In 1914 and 1939 two major Western European powers, Britain and France, found themselves fighting Germany because of crises that began in Eastern Europe, not directly because of their strategic interests closer to home. Contingency can frequently trump the logic of international relations.

By contrast, wars end »not on the moment« (157) according to Jörn Leonhard, but in at least four phases – preliminary negotiations, an armistice, the peace settlement itself, and the subsequent building of trust between the erstwhile belligerents. The longer the war and the greater the number of belligerents, the more complex the process. Leonhard reckons that the Thirty Years’ War could have ended in 1636 and that Ferdinand III of Austria favoured peace by 1641 but that not until 1645 did negotiations become serious. The ending of the war in Germany in 1648 required two treaties not one and, even then, France and Spain continued their own war, which had piggy-backed on that within Germany, until 1659. The process of ending the Thirty Years’ War took almost as long as the war itself.

Leonhard’s brief book, which reads as though it started life as a lecture series, advances ten theses about the manner in which peace is established. First, the process is determined by the war's nature, not in a military or Clausewitzian sense, but in political terms. If a government senses that its political legitimacy is dependent on victory it doubles down on its efforts, even if neutral intermediaries intervene to initiate negotiations. Second, few wars are settled by decisive battles. Instead, the effect of initial defeat prompts the loser to remobilise and so intensify and prolong the conflict. Third, a bad peace, by which Leonhard means a settlement which is unjust and vindictive, can also lengthen a war rather than end it because it provokes the party that is humiliated to renew hostilities. The effect of these military and political contingencies, fourthly, is that belligerents – once they are at war with one another – will continue to hope that something will change the equation in their favour, even when all the evidence suggests that the war is lost. Germany went on fighting after 1944, the year in which logic suggested it should have surrendered, as did the United States in Vietnam, despite the absence of any evidence that the intensification of its bombing campaign was persuading the North to negotiate. As a result, wars go on for longer than the balance of resources suggests is likely (Leonhard’s fifth thesis).

The last five theses address more directly the peace process itself. Some wars do not end with a peace treaty at all: the fighting stops with an armistice, which is only a temporary and conditioned pause, so leaving the war suspended, as in Korea in 1953 or in the war between Iran and Iraq in 1988. Whether a pause becomes permanent may depend on the magnanimity of the ostensible victor and the symbols he employs. Leonhard clearly approves of those who use the signals of conciliation over those of triumphalism. Appeals to military honour sweeten the taste of defeat. The peacemakers, even when victorious, need to be realistic about what they can achieve. In 1815, the pragmatism of Metternich and Talleyrand created a functioning international order and congress system, while Woodrow Wilson’s idealism and global ambition ensured he failed in 1919. In particular, whereas the negotiations that concluded the Napoleonic Wars were protracted and allowed plenty of opportunity for bilateral deals on the side, those in Paris in 1919 were too big and bureaucratic and too rushed. The armistice was collapsed into the peace process, not kept separate. Germany in 1919, unlike France in 1815, was presented with a fait accompli. Finally, only after the ink on the treaties is dry, does the real growth of trust and mutual respect begin. In the long run the defeated power may emerge as a winner. The breaking of the old order can enable both the resetting of the international system and the more rapid recovery of the humiliated nation, now freer of the incubus of the past than is the victor, who is hallowed by battlefield success and so encumbered by the myths that it generates.

In Über Kriege und wie man sie beendet, Leonhard focuses mostly on early modern and modern Europe. He has more to say about the Thirty Years War, the Napoleonic Wars and especially the First World War, where he relies heavily on his own earlier works on the war itself and on the Versailles treaty, than on other conflicts. His ten theses all make valid points and are well illustrated with examples, but they do not entertain exceptions to their arguments and so lack texture and nuance.

In mixing realism with liberalism, Leonhard takes the emotional force out of both war and peace. War hardens animosities on both sides, as we see today in Ukraine and Russia. It makes negotiation and reconciliation counterintuitive. In criticising French revanchism at Versailles in 1919, Leonhard fails to take into account the popular animosities of their own peoples which limited the options of democratically elected politicians – just as he underestimates the wilfulness of the Nazis in their exploitation both of the war guilt clause in the Versailles treaty and of the allies’ subsequent pursuit of reparations. The expectation that the defeated powers would pay an indemnity as the price of their humiliation was not a departure from the past, as he suggests, but had been hallowed repeatedly by Napoleon and – more relevantly – by Bismarck in 1871.

Absent too is the idealism inherent in the making of peace. Pacifism may struggle to make itself heard in the heat of war, but it still has political leverage. Was Woodrow Wilson naïve and misguided – as many European statesmen felt in 1919 and as many Americans believe today? Or was his vision the right one and his problem its execution and its timing? When Wilson arrived in Europe in January 1919, what he said was what many peoples on both sides of the war wanted to hear. And even had he tempered his vision with balance-of-power realism, could he have done so to any effect? Such a treaty would have stood even less chance of ratification in the United States and could not have snatched the moral high ground from the Bolsheviks, seemingly on the advance into Eastern and Central Europe.

Leonhard’s point seems to be that, just as one wages war as one must, not as one might like, so the same applies to peace. But, if so, peace remains the corollary of war and its achievement – rather than standing in its own right, with its own conditions and expectations – is contingent on how the war itself is conducted.