The word genocide was coined in 1944 by a Polish lawyer named Raphael Lemkin, who lost 49 members of his Jewish family in the Holocaust.

But it wasn’t the Nazis who first got him thinking about how to stop the intentional destruction of national, ethnic or religious groups. Decades earlier, when he was in college, he heard about the assassination of Talaat Pasha, one of the main organizers of the deportation and mass killing of Armenians in the Ottoman Empire, by an Armenian man who had survived it. The subsequent trial of the assassin opened his eyes to the suffering of the Armenian people.

“At that moment,” Lemkin wrote later in his autobiography, “my worries about the murder of the innocent became more meaningful to me. I didn’t know all the answers, but I felt that a law against this type of racial or religious murder must be adopted by the world.”

The Ottoman Empire killed an estimated 1.5 million Armenians during World War I. On Saturday, President Biden called it “genocide,” making him the first president to do so since Ronald Reagan. It’s a move that could further strain relations with U.S. ally Turkey.

The Ottoman Empire comprised many different ethnic and religious groups but was largely controlled by Muslims. In 1908, a group called the Young Turks seized control, first of a society called the Committee of Union and Progress, and then of the government. The CUP promised modernization, prosperity and secular, constitutional reforms.

At first, it seemed as though this vision included ethnic Armenians, most of whom were poor peasants on the eastern side of Anatolia (what is now Turkey). But over the next few years, the CUP grew increasingly focused on Turkish nationalism; by 1913, it was a full-on dictatorship.

When World War I broke out, Armenians found themselves physically on both sides of the battlefront between the Ottomans and the Russians. The Ottoman government drafted Armenian men to fight, but when the military suffered heavy losses, it blamed them on Armenians, accusing them of collaborating with the enemy. The Armenian soldiers were disarmed and murdered by Ottoman troops.

On April 24, 1915, the government arrested about 250 Armenian leaders and intellectuals. This is seen by many as the beginning of the massacre, and April 24 now marks Armenian Genocide Remembrance Day.