In what has become an almost annual exercise, Turkey has thrown a fit because someone has spoken the truth about its dark past. This time, it has pulled its ambassador from Berlin and threatened dire consequences over a resolution, passed overwhelmingly by the German Parliament on Thursday, declaring that the century-old massacre of Ottoman Armenians was a genocide. That is what Turkey does every time a foreign government dares to challenge its discredited claim that the Armenians perished in the cruel fog of World War I, and not in a premeditated attempt to eradicate a people. Germany’s claims to the contrary, Turkish legislators huffed in a statement, are “based on biased, distorted and various subjective political motives.”

No, it was a genocide, the first of the 20th century. Historians have established beyond reasonable doubt that as many as 1.5 million Armenians were deliberately killed or sent on death marches in 1915-16 by the disintegrating Ottoman Empire, fearful that they and other Christian minorities could side with Russia in the war.

For Armenians, millions of whom were left scattered around the world, gaining recognition that the slaughter was a genocide — a deliberate atrocity, and not collateral damage — has been a long and passionate national mission, which has resulted in formal recognition by more than 20 countries.

The Armenians are fully justified in their quest for a historical reckoning. But the more the world has recognized that, the more aggressively Turkey has stormed and shouted. A couple of years ago, when President Recep Tayyip Erdogan was still a relatively broad-minded prime minister, he seemed prepared to take a more conciliatory stance on the Armenian issue. It never happened, and the increasingly autocratic Mr. Erdogan warned Germany’s chancellor, Angela Merkel, in advance that relations with Germany — “bilateral, diplomatic, economic, trade, political and military” — would be damaged by the resolution.