In July 1915, the American ambassador to the Ottoman Empire sent Washington a harrowing report about the Turks’ “systematic attempt to uproot peaceful Armenian populations.” He described “terrible tortures, wholesale expulsions and deportations from one end of the Empire to the other accompanied by frequent instances of rape, pillage and murder, turning into massacre.” A month later, the ambassador, Henry Morgenthau — the grandfather of the Manhattan district attorney, Robert M. Morgenthau — warned of an “attempt to exterminate a race.”

The Young Turk nationalist campaign against the empire’s Armenian subjects was far too enormous to be ignored at the time. But decades of government-backed denial have created what amounts to a taboo in Turkey today. Instead of admitting genocide, Turkish officials contend the Armenians were a dangerous fifth column that colluded with Russia in World War I; many Armenians may have died, they say, but there was no organized slaughter. Turkish writers who challenge this line, like the novelists Orhan Pamuk and Elif Shafak, have risked prosecution for insulting Turkish identity. And on the diplomatic front, when Turkey should be polishing its credentials for eventual European Union membership, it is mired in historical fights; this May, for instance, it pulled out of a NATO military exercise to protest the Canadian prime minister’s acknowledgment of the genocide.

“A Shameful Act: The Armenian Genocide and the Question of Turkish Responsibility,” by Taner Akcam, is a Turkish blast against this national denial. A historian and former leftist activist now teaching at the Center for Holocaust and Genocide Studies at the University of Minnesota, Akcam is often described as the first Turkish scholar to call the massacres genocide, and his impressive achievement here is to shine fresh light on exactly why and how the Ottoman Empire deported and slaughtered the Armenians. He directly challenges the doubters back home, basing his powerful book on Turkish sources in the old Ottoman script — including the failed Ottoman war crimes tribunals held after World War I. Although he bolsters his case with material from the American, British and German archives, he writes that the remaining Ottoman records are enough to show that the ruling party’s central committee “did deliberately attempt to destroy the Armenian population.”

Akcam closely links the 1915 genocide with World War I. The Unionists, as the nationalist leaders were known, dreaded the partition of their empire by the European great powers. Not only did they suspect the Armenians of dangerous disloyalty, Akcam writes, but massacres of Muslims in Christian regions of the faltering empire before World War I had fostered a desire for vengeance.