Armenia on Thursday suspended ratification of peace accords with Turkey, setting back to square one US-backed efforts to bury a century of hostility between the neighbours.
Here are some details of the troubled history between the two countries, which share a border in the South Caucasus.
*Muslim Turkey closed its land border with Christian Armenia in 1993 in protest at Yerevan's backing for ethnic Armenian rebels fighting for control of Nagorno-Karabakh, a breakaway region of Azerbaijan populated mainly by ethnic Armenians.
*Turkey is a close ally of Muslim Azerbaijan, an oil and gas exporter. The two countries share close cultural and linguistic ties. Some 30,000 people were killed in the war, which ended in 1994 with Armenian forces occupying Nagorno-Karabakh and seven other, adjacent districts of Azerbaijan.
*Armenia has been angered by statements by Turkish Prime Minister Tayyip Erdogan saying ratification of peace accords between Turkey and Armenia would depend on Armenia reaching a deal with Azerbaijan.
*The peace accords made no mention of Nagorno-Karabakh.
*Azerbaijan says the frontier must stay shut until ethnic Armenian forces pull back, and has lashed out at Washington for backing the thaw.
*The Nagorno-Karabakh conflict has resisted more than 15 years of slow-paced international mediation. US, Russian and French mediators are pushing for agreement.
*As the Ottomans fought Russian forces in eastern Anatolia in the chaotic end of the Ottoman Empire during World War One, many Armenians formed partisan groups to assist the invading Russian army. On April 24, 1915, the Ottoman Empire arrested and killed hundreds of the Armenian intelligentsia.
*In May, Ottoman commanders began mass deportation of Armenians from eastern Anatolia. Thousands were marched towards Syria and Mesopotamia, and Armenians say some 1.5 million died, either in massacres or of starvation and exhaustion.
*The accords signed between the governments of Turkey and Armenia in October 2009 to establish diplomatic ties and open the border call for the creation of a commission of international experts to study the events.
*Armenia insists they should be declared a genocide, term used by some Western historians and foreign parliaments.
*President Serzh Sarksyan has said that securing international condemnation of the killings was a priority for his administration.
*Ankara strongly rejects the "genocide" description, saying large numbers of both Christian Armenians and Muslim Turks were killed during the break-up of the Ottoman Empire.
*Sarksyan said on Thursday his country has suspended ratification of a deal to mend ties with Turkey, but was not leaving the process.
*The process was already deadlocked before Thursday's decision, with each side accusing the other of trying to re-write the texts and setting new conditions.
*The Armenian decision - two days before the 95th anniversary of the killings - could be a bid by Yerevan to increase pressure on Turkey.
*Analysts and diplomats see little prospect of either side giving ground, as both governments face opposition at home, and in Armenia's case from its huge diaspora.
*Any accord between Armenia-Turkey could reverberate around the volatile South Causcasus, a region criss-crossed by pipelines carrying oil and gas to the West.
*In February, Azerbaijan warned that a "great war" in the South Caucasus was inevitable if Armenian forces did not withdraw. Azerbaijan frequently makes threats to take Nagorno back by force, but tensions have increased in the past year.
*US President Barack Obama will issue a statement to mark the anniversary of the Armenian massacres on April 24.