US President Joe Biden on Saturday recognised the 1915 killings of hundreds of thousands of Armenians by Ottoman forces as genocide, a move immediately welcomed by Armenia and rejected by Turkey as a "grave mistake".
Mr Biden became the first US president to use the word genocide in a customary statement on the anniversary, a day after informing Turkish President Recep Tayyip Erdogan that he would go ahead with this step and seeking to limit the expected furore from the Nato ally.
"We remember the lives of all those who died in the Ottoman-era Armenian genocide and recommit ourselves to preventing such an atrocity from ever again occurring," Mr Biden said.
"And we remember so that we remain ever-vigilant against the corrosive influence of hate in all its forms."
There were scattered scenes of celebration in the Armenian capital Yerevan as news of the declaration broke on Saturday evening.
Near the central Opera Square, occasional cheers and blaring of car horns were heard after people leant that the US had finally recognised the genocide.
The statement is a massive victory for Armenia and its extensive diaspora.
Starting with Uruguay in 1965, nations including France, Germany, Canada and Russia have recognised the genocide but a US statement has been a paramount goal that proved elusive under other presidents until Mr Biden.
Armenian Prime Minister Nikol Pashinyan welcomed Mr Biden's statement.
"The people of Armenia and Armenians all over the world perceived with great enthusiasm and welcomed your message," he said.
"The acknowledgement of the Armenian Genocide is important not only as a tribute paid to the 1.5 million innocent victims, but also in terms of preventing the recurrence of similar crimes against mankind."
Mr Biden's statement coincided with Armenian Genocide Remembrance Day on Saturday.
Turkey's Foreign Ministry called on Mr Biden to "correct this grave mistake", saying his statement "will open a deep wound that undermines our mutual trust and friendship".
"It is clear that the said statement does not have a scholarly and legal basis, nor is it supported by any evidence," the ministry said.
Opposition Republican People’s Party (CHP) spokesman Faik Oztrak said the US statement would “go down in history as a great mistake.” He added that it would cause “irreparable wounds” to Turkey-US relations, and was a missed chance to improve ties between Turkey and Armenia.
A senior US official said Mr Biden made the decision "in a very principled way focused on the merits of human rights, and not for any reason beyond that, including placing blame".
Biden statement places incremental pressure on Turkey
Benyamin Poghosyan, chairman of the Centre for Political and Economic Strategic Studies in Yerevan, said Mr Biden's statement was highly significant.
"Turkey is still far off but each time a new country recognises the genocide we come closer to Turkey recognising it. Of course, US recognition, as the most powerful country in the world, sends a strong signal to other states," he told The National.
Mr Biden took office vowing to put a new focus on human rights and democracy after his volatile predecessor Donald Trump, who befriended authoritarians and, despite breaking plenty of foreign policy precedents, declined to recognise the Armenian genocide.
Explaining Mr Biden's thinking, the administration official also alluded to the Democratic president's outspokenness on systemic racism in the US.
Across the world, "people are beginning to acknowledge and address and grapple with the painful historical facts in their own countries. It's certainly something that we are doing here in the United States," she said.
An estimated 1.5 million Armenians were killed from 1915 to 1917 during the waning days of the Ottoman Empire, which suspected the Christian minority of conspiring with adversary Russia in the First World War.
Armenian populations were rounded up and deported into the desert of Syria on death marches where many were shot, poisoned or fell victim to disease, according to accounts at the time by foreign diplomats.
Turkey, which emerged as a secular republic from the ashes of the Ottoman Empire, acknowledges that 300,000 Armenians may have died but strongly rejects that it was genocide, saying they perished in strife and famine in which many Turks also died.
Mr Erdogan on Thursday told advisers to "defend the truth against those who back the so-called 'Armenian genocide' lie", with his foreign minister warning that the United States would set back relations.
Recognition has been a top priority for Armenia and Armenian-Americans, with calls for compensation and property restoration over what they call Meds Yeghern – the Great Crime.
Mr Biden's statement was also expected to heighten appeals from Armenia for greater US support against Turkish-backed neighbour Azerbaijan, which last year humiliated Armenia by taking back territories seized in the 1990s.
But Mr Biden, whose call to Mr Erdogan to inform him of the genocide recognition was their first conversation since the US leader took office three months ago, has signalled he hopes for limited diplomatic impact.
Mr Biden and Mr Erdogan agreed in their call to meet in June on the sidelines of a Nato summit in Brussels, officials said.
Mr Biden has kept Mr Erdogan at arm's length – a contrast with Mr Trump, whom the Turkish leader reportedly found so amenable that he would call him directly on his phone on the golf course.
The US Congress in 2019 voted overwhelmingly to recognise the Armenian genocide but the Trump administration made clear that the official US line had not changed.
Biden recognition of genocide a departure from Obama regime
Former president Barack Obama, under whom Mr Biden served as vice president, danced around the issue by quoting pre-election statements he made recognising the genocide and resisted pressure for a statement on the centennial in 2015.
Alan Makovsky, an expert on Turkey at the left-leaning Centre for American Progress, said that the 2019 congressional resolution had "no discernible impact" on US-Turkey relations and paved the way for Mr Biden to go ahead.
"We've seen through experience that concern about Turkey's reaction was always overblown," he said.
"Turkey will raise a rhetorical fuss for a few days and perhaps delay acting on some routine requests from the US military."
Tensions have risen with Turkey in recent years over its purchase of a major air defence system from Russia – the chief adversary of Nato – which under US law could trigger sanctions.
Turkey has also infuriated much of the US political establishment with its incursions against US-allied Kurdish fighters in Syria who helped fight ISIS but are linked to militants inside Turkey.
Mr Biden before taking office called Mr Erdogan an autocrat and voiced support for Turkey's opposition.