US President Joe Biden said on Saturday that the 1915 massacres and forced deportation of Armenians in the Ottoman Empire constituted genocide, a move that infuriated Turkey and further strained frayed ties between the two NATO allies.
Here is some background to the issue.
In the late 19th century the Ottoman Empire's roughly two million Armenians began to assert nationalist aspirations.
Repression by Ottoman irregulars, mainly Kurds, led to the massacre of tens of thousands of Armenians in 1894-1896 in eastern Anatolia, now eastern Turkey. Several thousand more were killed in Constantinople, now Istanbul, in August 1896 after Armenian militants seized the Ottoman Bank.
As the Ottomans fought Russian forces in eastern Anatolia in World War One, many Armenians formed partisan groups to assist the invading Russian armies.
On April 24, 1915, the Ottoman Empire arrested and ultimately killed hundreds of the Armenian intelligentsia.
In May 1915, Ottoman commanders began mass deportation of Armenians from eastern Anatolia. Thousands were marched south towards Syria and Mesopotamia. Armenians say some 1.5 million died in massacres or of starvation and exhaustion in the desert.
What does turkey say?
The Turkish republic, established in 1923 after the Ottoman empire collapsed, has always denied there was a systematic campaign to annihilate Armenians.
It says that thousands of Turks and Armenians died in inter-ethnic violence as the empire started to fall apart and fought a Russian invasion of its eastern provinces during World War One.
How are Turkish-Armenian relations now?
Seeking to bury a century of hostility, the two countries signed a peace accord in 2009 which called for the creation of a commission of international experts to study the 1915 killings, which Armenia insisted should be declared a genocide.
They agreed to establish diplomatic ties and open their border, subject to parliamentary approval of the deal. But Yerevan and Ankara accused each other of trying to re-write the texts, and within six months the ratification was suspended.
Armenia formally scrapped the deal in 2018.
Last year, Turkey strongly supported Azerbaijan in its six-week conflict with Armenia over the enclave of Nagorno-Karabakh, when Azeri forces recaptured territory from Armenian troops.
What do other countries say?
The 1915 killings have been recognised as genocide in dozens of countries.
French President Emmanuel Macron and Italy's lower house of parliament officially declared in 2019 that the events constituted genocide. Macron decreed that April 24 should be a day of annual commemoration.
In the same year, both chambers of the US Congress passed resolutions saying the US should commemorate the killings as genocide.