Pessimism is a useful prism through which to view the affairs of states. Their ambition to gain, retain, and project power is never sated. Optimism, toward which Americans are generally inclined, leads to rash predictions of history’s ending in global consensus and the banishment of war. Such rosy views accompanied the end of the Cold War. They were also much in evidence a century ago, on the eve of World War I.

Then, as now, Europe had lived through a long period of relative peace, after the end of the Napoleonic Wars. Then, too, rapid progress in science, technology, and communications had given humanity a sense of shared interests that precluded war, despite the ominous naval competition between Britain and Germany. Then, too, wealthy individuals devoted their fortunes to conciliation and greater human understanding. Rival powers fumed over provocative annexations, like Austria-Hungary’s of Bosnia-Herzegovina in 1908, but world leaders scarcely believed a global conflagration was possible, let alone that one would begin just six years later. The very monarchs who would consign tens of millions to a murderous morass from 1914 to 1918 and bury four empires believed they were clever enough to finesse the worst.

The unimaginable can occur. That is a notion at once banal and perennially useful to recall. Indeed, it has just happened in Crimea, where a major power has forcefully changed a European border for the first time since 1945. Russia’s act of annexation and its evident designs on eastern Ukraine constitute a reminder that NATO was created to protect Europe after its pair of 20th-century self-immolations. NATO’s core precept, as the Poles and other former vassals of the Soviet empire like to remind blithe western Europeans, is Article 5, by which the Allies agreed that “an armed attack against one or more of them in Europe or North America shall be considered an attack against them all,” triggering a joint military response. This has proved a powerful deterrent against potential adversaries. Vladimir Putin, the Russian president, has been most aggressive in the no-man’s-lands of Georgia and Ukraine, nations suspended between East and West, neither one a member of NATO. Had Ukraine been a member of NATO, the annexation of Crimea would have come only at the (presumably unacceptable) price of war. Article 5, until demonstrated otherwise, is an ironclad commitment.

When a 19-year-old Bosnian Serb nationalist, Gavrilo Princip, assassinated the heir to the Austro-Hungarian throne in Sarajevo, on June 28, 1914, he acted to secure Serbia’s liberty from imperial dominion. He could not have known that within weeks, Austria-Hungary would declare war on Serbia, goading Russia (humiliated in war a decade earlier by Japan) to mobilize in defense of its Slavic ally, which caused the kaiser’s ascendant Germany to launch a preemptive attack on Russia’s ally France, in turn prompting Britain to declare war on Germany.

Events cascade. It is already clear that the nationalist fervor unleashed by Putin after a quarter century of Russia’s perceived post–Cold War decline is far from exhausted. Russians are sure that the dignity of their nation has been trampled by an American and European strategic advance to their border dressed up in talk of democracy, the rule of law, and human rights. Whether this is true is irrelevant; they believe it. National humiliation, real or not, is a tremendous catalyst for war. That was the case in Germany after the Treaty of Versailles imposed reparations and territorial concessions; so, too, in Serbia more than 70 years later, after the breakup of Yugoslavia, a country Serbia had always viewed as an extension of itself. Russia, convinced of its lost greatness, is gripped by a Weimar neurosis resembling Germany’s post–World War I longing for its past stature and power. The Moscow-backed separatists taking over government buildings in eastern Ukraine and proclaiming an independent “Donetsk People’s Republic” demonstrate the virulence of Russian irredentism. Nobody can know where it will stop. Appetite, as the French say, grows with eating. ...