As many as 1.5 million Armenians were killed during the systematic deportation and annihilation of the Christian minority under Ottoman rule.
While there is general agreement that hundreds of thousands of Armenians died during their mass deportation from eastern Anatolia to the Syrian desert in 1915-16, what is disputed is the degree to which the deaths were premeditated.
Every year on April 24, the Armenian state commemorates the killings, which it refers to simply as the Genocide. This label is rejected by Turkey, the successor state of the Ottoman Empire.
The controversy heated up last year, which marked the 100th anniversary of the beginning of the massacre.
Turkish President Recep Tayyip Erdogan said there was no systematic attempt to destroy the Christian Armenian people and vowed never to recognize the killings as genocide.
His country criticized the European Parliament for passing a resolution that called on Ankara to recognize the Armenian genocide, and Pope Francis for describing the Armenians as the victims of "the first genocide of the 20th century."
Germany's decision last year to present a parliamentary resolution that described Turkish actions against Armenians in 1915-16 as genocidal also caused tensions with Turkey.
In a speech, President Joachim Gauck acknowledged that German military advisers helped to coordinate the mass deportations and that his country was "in some cases complicit" in the genocide.
Germany was an ally of the Ottoman Empire during World War I.