In the 1820s, as it waged a war of independence against the Ottoman Empire, Greece became an early symbol of escape from the prison house of empire. For philhellenes, its resurrection represented the noblest of causes. “In the great morning of the world,” Shelley wrote in “Hellas,” his poem about the country’s struggle for independence, “Freedom’s splendor burst and shone!” Victory would mean liberty’s triumph not only over the Turks but also over all those dynasts who had kept so many Europeans enslaved. Germans, Italians, Poles and Americans flocked to fight under the Greek blue and white for the sake of democracy. And within a decade, the country won its freedom.

Over the next century, the radically new combination of constitutional democracy and ethnic nationalism that Greece embodied spread across the continent, culminating in “the peace to end all peace” at the end of the First World War, when the Ottoman, Hapsburg and Russian empires disintegrated and were replaced by nation-states.

In the aftermath of the First World War, Greece again paved the way for Europe’s future. Only now it was democracy’s dark side that came to the fore. In a world of nation-states, ethnic minorities like Greece’s Muslim population and the Orthodox Christians of Asia Minor were a recipe for international instability. In the early 1920s, Greek and Turkish leaders decided to swap their minority populations, expelling some two million Christians and Muslims in the interest of national homogeneity. The Greco-Turkish population exchange was the largest such organized refugee movement in history to that point and a model that the Nazis and others would point to later for displacing peoples in Eastern Europe, the Middle East and India.

It is ironic, then, that Greece was in the vanguard of resistance to the Nazis, too. In the winter of 1940-41, it was the first country to fight back effectively against the Axis powers, humiliating Mussolini in the Greco-Italian war while the rest of Europe cheered. And many cheered again a few months later when a young left-wing resistance fighter named Manolis Glezos climbed the Acropolis one night with a friend and pulled down a swastika flag that the Germans had recently unfurled. (Almost 70 years later, Mr. Glezos would be tear-gassed by the Greek police while protesting the austerity program.) Ultimately, however, Greece succumbed to German occupation. Nazi rule brought with it political disintegration, mass starvation and, after liberation, the descent of the country into outright civil war between Communist and anti-Communist forces.

Only a few years after Hitler’s defeat, Greece found itself in the center of history again, as a front line in the cold war. In 1947, President Harry S. Truman used the intensifying civil war there to galvanize Congress behind the Truman Doctrine and his sweeping peacetime commitment of American resources to fight Communism and rebuild Europe. Suddenly elevated into a trans-Atlantic cause, Greece now stood for a very different Europe — one that had crippled itself by tearing itself apart, whose only path out of the destitution of the mid-1940s was as a junior partner with Washington. As the dollars poured in, American advisers sat in Athens telling Greek policy makers what to do and American napalm scorched the Greek mountains as the Communists were put to flight.

European political and economic integration was supposed to end the weakness and dependency of the divided continent, and here, too, Greece was an emblem of a new phase in its history. The fall of its military dictatorship in 1974 not only brought the country full membership in what would become the European Union; it also (along with the transitions in Spain and Portugal at the same time) prefigured the global democratization wave of the 1980s and ’90s, first in South America and Southeast Asia and then in Eastern Europe. And it gave the European Union the taste for enlargement and the ambition to turn itself from a small club of wealthy Western European states into a voice for the newly democratic continent as a whole, extending far to the south and east.

And now today, after the euphoria of the ’90s has faded and a new modesty sets in among the Europeans, it falls again to Greece to challenge the mandarins of the European Union and to ask what lies ahead for the continent. The European Union was supposed to shore up a fragmented Europe, to consolidate its democratic potential and to transform the continent into a force capable of competing on the global stage. It is perhaps fitting that one of Europe’s oldest and most democratic nation-states should be on the new front line, throwing all these achievements into question. For we are all small powers now, and once again Greece is in the forefront of the fight for the future.