At the risk of infuriating Turkey, President Biden is set to formally announce on Saturday that the United States regards the killing of 1.5 million Armenians by Turks more than a century ago to be a genocide — the most monstrous of crimes.

Mr. Biden would be the first American president to make such an announcement, breaking with predecessors who did not wish to antagonize Turkey, a NATO ally and a strategically pivotal country straddling Europe and the Middle East.

The expected announcement, which Mr. Biden had signaled when he was a candidate last year, has been welcomed by Armenians and human rights advocates. It carries enormous symbolic weight, equating the anti-Armenian violence with atrocities on the scale of those committed in Nazi-occupied Europe, Cambodia and Rwanda.

Use of the term is a moral slap at President Tayyip Recep Erdogan of Turkey, a fervent denier of the genocide. He has fulminated at other leaders, including Pope Francis, for describing the Armenian killings that way.

a crime under international law.

Cambodia, the 1994 genocide in Rwanda, and atrocities including genocide in the former Yugoslavia.

The International Criminal Court, which was created in 2002 in part to prosecute such crimes, has only one pending genocide case — Omar Hassan Ahmad al-Bashir, former president of Sudan, who is wanted on two warrants for crimes including genocide in the Darfur region between 2003 and 2008. The court cannot prosecute crimes committed before its inception.

The International Court of Justice, the highest court of the United Nations, ruled in January 2020 that Myanmar must take action to protect Rohingya Muslims, who have been killed and driven from their homes in what the country’s accusers have called a campaign of genocide. The ruling, which has no enforcement power, was the outcome of a lawsuit filed on behalf of Muslim countries that wanted the court to condemn Myanmar for violating the genocide treaty.

What was the Armenian genocide?

The violence against Armenians began during the breakup of the Ottoman Empire, the predecessor of modern Turkey, which included an area that is now Armenia, a landlocked country ringed by Turkey, Georgia, Azerbaijan and Iran.

Starting in 1915, the Ottomans, aligned with Germany in World War I, sought to prevent Armenians from collaborating with Russia and ordered mass deportations. As many as 1.5 million ethnic Armenians died from starvation, killings by Ottoman Turk soldiers and the police, and forced exoduses south into what is now Syria and elsewhere in the Middle East.

denial of genocide is ingrained into Turkish society. Writers who have dared to use the term have been prosecuted under Section 301 of Turkey’s penal code, which bans “denigrating Turkishness.” The denial is taught at an early age, with school textbooks calling the genocide a lie, describing the Armenians of that period as traitors and declaring the actions by the Ottoman Turks as “necessary measures” against Armenian separatism.

Why have U.S. presidents refrained from calling the Armenian killings a genocide?

Some have come close. President Ronald Reagan tangentially referred to the “genocide of the Armenians” in an April 22, 1981, statement commemorating the liberation of the Nazi death camps.

But American presidents have generally avoided describing the killings this way to avoid any backlash from Turkey that would endanger its cooperation in regional conflicts or diplomacy.

What has changed?

As a presidential candidate, Mr. Biden signaled his intentions a year ago in a speech on April 24, Armenia’s official day of remembrance of the genocide. He used the term “Armenian genocide” and asserted that “we must never forget or remain silent about this horrific and systematic campaign of extermination.” And in recent years, bipartisan anger toward Mr. Erdogan has grown. In 2019, the House and Senate passed resolutions calling the Armenian killings a genocide.

As vice president in the Obama administration, Mr. Biden never enjoyed an easy relationship with Mr. Erdogan, an autocratic leader who gave him an icy reception in August 2016. The two met a month after the failed coup in Turkey that Mr. Erdogan blamed on a Turkish cleric living in exile in the United States.