The massacre of as many as 1.5 million Armenians by the Ottoman Empire during World War I is commemorated each year on April 24.

Armenians refer to the mass killings as the Armenian genocide — a term that Turkey rejects and which the United States had for decades refrained from using.

That changed Saturday, when President Biden recognized it as a “genocide” in an annual Remembrance Day declaration.

Here’s what this could mean.

Why does Turkey oppose the term ‘genocide’?

The 1948 United Nations convention on genocide defines it as the crime of acting “to destroy, in whole or in part, a national, ethnic, racial or religious group.”

Historians estimate that around 1.5 million Armenian Christians were killed during massacres and deportation campaigns carried out by the Ottoman Empire beginning in 1915. Many use the word genocide to describe the events.

But Turkey, the modern-day successor of the Ottoman Empire, rejects this allegation. Successive Turkish leaders have maintained that while some atrocities did occur, the deaths and persecution were nothing to the degree that Armenia and its supporters claim.

Instead, Turkey says that some 300,000 Armenians died during World War I as a result of the civil war and internal upheavals that consumed the Ottoman Empire as it splintered. In addition to Armenian Christians, Turkey says that many Muslim Turks died during this period.

Armenians today are considered among the world’s most dispersed peoples, according to the BBC. The mass killings more than a century ago are a defining moment for Armenia and its diaspora.

But for Turkey, the term genocide threatens the story it tells about the founding of its modern nation-state. Writers who use the term have been prosecuted under Article 301 of Turkey’s penal code, which criminalizes “insulting Turkishness.”

Why has the United States refrained from using the term?

Former presidents Barack Obama and Donald Trump, among others, did not use the word to avoid angering Turkey. Ankara is a longtime U.S. ally and a strategic NATO member, sharing a neighborhood with Russia and the Middle East. More recently, it was part of the fight against the Islamic State.

Ankara has repeatedly warned Washington that changing its stance would threaten U.S.-Turkish relations and shared interests such as an agreement that allows the United States access to a military base in the south of the country.

Turkey frequently complains when other countries use the term genocide. Some 20 countries do, including France and Canada, while other key U.S. allies, including Israel and Britain, do not.

In 2019, Congress passed a resolution calling the killings a genocide. The move infuriated Turkish President Recep Tayyip Erdogan, and Trump officially rejected it.

Obama had pledged to formally recognize the Armenian genocide when he first ran in 2008, but by the end of his eight years in office, he had not done so.

Samantha Powers, Obama’s ambassador to the United Nations and now Biden’s nominee to lead the U.S. Agency for International Development, said during a 2018 interview that she and others in the administration were “played a little bit” by Erdogan.

“Every year there was a reason not to,” Ben Rhodes, a deputy national security adviser in the Obama administration, said in the same interview in 2018. “Turkey was vital to some issue that we were dealing with, or there was some dialogue between Turkey and the Armenian government about the past.”

“Frankly, here’s the lesson, I think, going forward: Get it done the first year, you know, because if you don’t, it gets harder every year in a way,” he said.

What may be the impact of the change?

Biden, who as Obama’s vice president was presumably privy to these discussions, similarly promised to use the word while campaigning.

“If elected, I pledge to support a resolution recognizing the Armenian Genocide and will make universal human rights a top priority for my administration,” Biden said in a statement marking Armenia’s Remembrance Day last year.

Now as president, Biden’s follow-through comes after four tense years of relations between Trump and Erdogan. He might also have calculated that taking a stand on a historical event could be a relatively easy way to begin retooling his approach to foreign policy and human rights.

“The American people honor all those Armenians who perished in the genocide that began 106 years ago today,” Biden said in a statement Saturday. “Let us renew our shared resolve to prevent future atrocities from occurring anywhere in the world. And let us pursue healing and reconciliation for all the people of the world.”

On April 24, Armenians laid flowers at the country's memorial to the victims of massacres, deportations and forced marches under the rule of Ottoman Turkey. (Jonathan Baran/AP)

How have Turkey and Armenia’s supporters responded?

Erdogan briefly weighed in Thursday, before Biden’s announcement, saying that Turkey will continue to defend its history of what Turkish media called “the events of 1915.”

On Saturday, Turkey’s foreign minister, Mevlut Cavusoglu, swiftly condemned Biden’s remarks.

“We entirely reject this statement,” he wrote on Twitter. “We have nothing to learn from anybody on our own past. Political opportunism is the greatest betrayal to peace and justice.”

Many Armenian American activists had been pushing Biden to fulfill his campaign promise. On Wednesday, over 100 members of Congress sent a letter to Biden urging him to do so.

“We join with the proud Armenian American community and all of those who support truth and justice in asking that you clearly and directly recognize the Armenian Genocide,” they wrote.

Armenian Prime Minister Nikol Pashinyan praised the move Saturday in a letter to Biden. He said it was welcomed by “Armenians all over the world” and called it important for Armenia’s security, Reuters reported.