Last week the U.S. House of Representatives passed H.R. 296, a resolution “recognizing and condemning the Armenian Genocide, the killing of 1.5 million Armenians by the Ottoman Empire from 1915 to 1923.” A total of 405 representatives voted for the bill, while only 11 voted against it and three voted “present.”

Here is what you should know about one of the most horrific atrocities against Christians in modern times.

1. The Armenian people have lived in the Caucasus region of Eurasia for thousands of years. The kingdom of Armenia was even the first nation in the world to make Christianity its official religion in the fourth century. But during the 15th century, Armenia was absorbed into the Ottoman Empire, which controlled much of Southeast Europe, Western Asia, and North Africa, and whose rulers were Muslim.

2. In 1914, the Ottoman Empire entered World War I on the side of Germany and Austria-Hungary. The Ottoman authorities began a propaganda campaign portraying the Christian Armenians as being “in league with the enemy.” On April 24, 1915, hundreds of Armenian community leaders and intellectuals suspected of being hostile to the Ottoman government were rounded up in Constantinople (present-day Istanbul). Many of them ended up deported or assassinated. That date is now known as Red Sunday, and is commemorated as Genocide Remembrance Day by Armenians around the world.

3. The next month the Ottoman authorities passed the Temporary Law of Deportation (“Tehcir Law”) authorizing the deportation of the Armenian population. The government forced the population to march to concentration camps in desert regions in what is today northern and eastern Syria, northern Saudi Arabia, and Iraq. Scholars estimate that 600,000 to more than 1,000,000 Armenians were slaughtered or died on the marches.

4. The Ottoman authorities implemented a plan to systematically remove and kill all Armenian men who could resist. As one U.S. ambassador to the Ottomans wrote, “[T]hroughout the Turkish Empire a systematic attempt was made to kill all able-bodied men, not only for the purpose of removing all males who might propagate a new generation of Armenians, but for the purpose of rendering the weaker part of the population an easy prey.” The government disarmed the Armenian soldiers in the Turkish army and transformed them into chain-gangs. “Those Armenian soldiers that did not perish from hard labor were murdered,” Jeremiah Harrelson says. “Then more young Armenian men were drafted into the Army and similarly executed.”

5. During the genocide, Armenian women were subjected to rape, kidnapping, sex slavery, and forced re-marriage. Reporting on the massacre, the British Consul recorded that “many other disgusting barbarities are said to have been committed, such as ripping open pregnant women, [and] tearing children to pieces by main force.” Additionally, “some sixty young women and girls were driven into a church, where the soldiers were ordered to do as they liked with them and afterwards kill them, which order was carried out.” At the concentration camps, guards were reported to have, “[v]ery often . . . violated eight or ten-year-old [Armenian] girls, and as a consequence many would be unable to walk, and were shot.” Historian Richard G. Hovannisian notes that the practice of rape was “more or less universal,” and that Armenians “were often killed in festivals of cruelty which involved rape and other forms of torture.”

6. The consensus among historians is that there were about 2 million Armenians living in the Ottoman Empire in 1915. However, the genocide also included some of the 1.8 million Armenians living in the Caucasus under Russian rule. The number of Armenians believed to have been killed between 1915 and 1917 is 1.5 million (Turkey claims that only 300,000 died during this period due to “war and disease”). By 1922 there were fewer than 400,000 Armenians remaining in the region.

7. The Armenian Genocide is considered one of the first genocides of the 20th century. The term “genocide” was coined in the 1940s by Raphael Lemkin as a combination of the root words genos (Greek for family, tribe, or race) and –cide (Latin for killing). Lemkin, a Polish lawyer who emigrated to the United States in 1941, developed the word, in part, to describe the atrocities suffered by the Armenians in the Ottoman Empire. “I became interested in genocide because it happened so many times,” Lempkin said in 1949. “First to the Armenians, then after the Armenians, Hitler took action.”

8. In a speech to Nazi commanders of Germany’s armed forces, Adolf Hitler justified the order to invade Poland and slaughter the Polish people (he gave orders “to send to death mercilessly and without compassion, men, women, and children of Polish derivation and language”) by pointing out that there were few repercussions from the genocide of the Armenians. As Hitler rhetorically asked, “Who, after all, speaks today of the annihilation of the Armenians?”

9. Despite the atrocities meeting the definition of a genocide (Article Two of the UN Convention on Genocide of December 1948 describes genocide as carrying out acts intended “to destroy, in whole or in part, a national, ethnic, racial or religious group”) using the phrase “Armenian genocide” has been disputed. About 30 countries—including Canada, France, Italy, Russia, and Vatican City—recognize the genocide against the Armenians. Some countries, such as the UK and Israel, use different terminology to describe the events. Although several official U.S. documents describe the events as genocide, the U.S. government has not yet officially recognized the Armenian Genocide. The only modern U.S. president to specifically call it a genocide was Ronald Reagan.