For decades, U.S. presidents have avoided calling the World War I-era mass killing of Armenians by Ottoman Turkish forces an act of genocide.Now, U.S. lawmakers expect President Biden to make that declaration on Saturday as Armenians mark the anniversary of the atrocities. News reports indicate that while the move is likely, Biden has not made a final decision.The possible declaration would be hailed by Armenian communities, lawmakers and human rights advocates who have lobbied for it. But it would also damage already strained ties with Turkey.Although some Turkish leaders have at times voiced regret for the killings, Turkey denies that they constitute genocide and fiercely opposes anyone using the term to describe the period.President Recep Tayyip Erdogan, in a statement Thursday in anticipation of Biden's announcement, said Turkey "will continue to defend truths against the so-called Armenian genocide lie and those who support this slander with political motivations."Many historians, however, agree that what the Ottoman Turkish forces did to Armenians amounts to genocide.Dueling commemorationsThis much is known: Up to 1.5 million Armenians were killed or deported in the violence unleashed by Ottoman Turks starting on April 24, 1915.Armenians, along with many historians and European countries, have called it the first genocide of the 20th century. Turkey suppressed accounts of the killings for decades, and to this day staunchly rejects the label of genocide.In the Military Museum in Istanbul, the room devoted to "Turkish-Armenian relations" is filled with historical photographs, not one of which depicts a slain Armenian — only the bodies of Turkish soldiers that Turkey says were tortured and killed by "Armenian gangs."Modern Turkey, which emerged following the collapse of the Ottoman Empire, has never accepted the general consensus about the Armenian genocide. It prefers to celebrate a different event that took place a day later, on April 25,1915: the victory over Allied forces at World War I's Battle of Gallipoli.In 2015, Turkey moved up a huge centennial celebration of the Gallipoli victory to April 24 in what looked to critics like a transparent effort to drown out ceremonies focused on the Armenian killings.The backgroundThe Ottoman Empire once covered parts of Europe, North Africa and the Middle East and was home to Turks, Kurds, Armenians and many others. But by the start of World War I in 1914, it was crumbling. A few years earlier, a group of young army officers — named the Young Turks — seized power. And in World War I, they sided with the Central Powers — Germany and the Austro-Hungarian Empire — against the Allies, Britain, France and Russia.Historian Eugene Rogan, author of The Fall of the Ottomans, told NPR's Steve Inskeep in 2015 that the Ottomans crossed into Russia thinking they might be able to strike a blow. Instead, they lost. There had been massacres of Armenians in the past, but with the loss to the Russians, he said, the Ottomans began to question the loyalties of the Armenians.He added: "What happened was a small number of [Armenian] militants who did cross over to the Russian side, who did actively try and recruit Armenians to support the Russian cause, made life extremely dangerous for the majority of Armenian civilians who basically had no fight with anyone, did not wish to be drawn into any war and found themselves under tremendous pressure; soldiers who, suspected by their Turkish comrades, begin to get shot down."The Ottomans' ruling Committee of Union and Progress and government officials planned to relocate the Armenians forcibly from Anatolia, where they lived, bordering Russia, to the Arab parts of the empire, where they were deemed to be less of a threat. But Rogan said the plans for the Armenians went beyond those that were written down. He added: "It was through testimony presented in trials the Ottomans convened after the war that we now know that the Committee of Union and Progress agreed to give, orally, orders for the extermination of Armenians: that men and women would be separated at the moment of departing their villages, that the men would be massacred and that the women would be marched under conditions in which only a fraction of them would survive. "And the theory that most Turkish scholars of the genocide are putting forward was that the Ottoman plan was to reduce the demographic profile of the Armenians so that they would not exceed 5% to 10% in any given province. It wasn't ... to try and eliminate the Armenians in their entirety, but it was to make sure that the Armenians would never constitute a critical mass to seek separation for the Ottoman Empire as an independent Armenian state."Earlier violence against ArmeniansArmenians in the Ottoman Empire were targeted even in the 19th century, but historians don't call those events a genocide. The reason, writer Peter Balakian told NPR's Robert Siegel in 2015, was that the earlier killings were "putative — they were punishments for Armenian progressive reform movement. They weren't designed to exterminate the entire population or rid the Ottoman Empire of its Armenian population, but they begin a very important process of devaluing and dehumanizing this ethnic minority group."Here's what he said was different about the events of 1915: "I think that the Ottoman government's final solution for the Armenian people of Turkey represented a shift in organized, state-planned mass killing. The Ottoman government was able to expedite its mass killing of a targeted minority population in a concentrated period of time. So it's important to realize that the Ottoman government murdered more than a million Armenians between 1915 and 1916 alone — perhaps 1.2 million is the number you come to by the end of the summer of 1916."The U.S. viewThe U.S., an ally of Turkey, has historically called the World War I-era killings an atrocity despite years of lobbying by the Armenian community in the United States.Although former President Ronald Reagan referred to "the genocide of the Armenians" in a 1981 proclamationremembering victims of the Nazi Holocaust, his predecessors and successors have fallen short of that description.Then in late 2019, Congress passed a resolution that lawmakers said "recognizes the Armenian genocide on behalf of the U.S. government," although the Trump administration refused to support the policy change.Biden pledged his support for that recognitionwhen he was a candidate last year, and he pushed for it as a senator.Under U.S. law — including legislation introduced in 1987 byBiden, then a senator — genocide refers to killing, injury, torture or other acts "with the specific intent to destroy, in whole or in substantial part, a national, ethnic, racial, or religious group."The term wasn't around at the time of the killings by Ottoman Turks. It was coined in 1944 by Raphael Lemkin, a Polish lawyer, who combined the Greek word genos, meaning race or family, with the suffix "-cide," which comes from the Latin for killing, to describe the events of the Holocaust and previous instances in history.As a teenager, Lemkin was drawn to the story of what happened to the Armenians in the Ottoman Empire after reading about a survivor of the atrocities. And in interviews in the 1940s he described the events as the Armenian genocide.The United States Holocaust Memorial Museum in Washington, which describes the events as a genocide, sees Lemkin's "early exposure to the history of Ottoman attacks against Armenians, anti-Semitic pogroms, and other cases of targeted violence as key to his beliefs about the need for the protection of groups under international law. Inspired by the murder of his own family during the Holocaust, Lemkin tirelessly championed this legal concept until it was codified in the United Nations Convention on the Prevention and Punishment of the Crime of Genocide in 1948."A previous version of this story, written by Krishnadev Calamur in Washington, was published on April 24, 2015, with the headline: "A Century After Atrocities Against Armenians, An Unresolved Wound." Peter Kenyon, reporting from Istanbul, wrote this update to reflect developments, including expectation that President Biden could recognize the massacres as genocide. Copyright 2021 NPR. To see more, visit https://www.npr.org.