When Ottoman Sultan Mehmed V declared "holy war" on Britain, France and Russia on November 24, 1914, his five-century-old empire was already in decline and had lost most of its European territory.
Convinced that Germany, an ally, was destined for a speedy victory, the empire's governing "Young Turks" movement saw the war as a chance to consolidate its grip on power, block the economic rise of London and Paris, and reclaim central Asia.
The Ottoman army inflicted a brutal defeat on British and French forces on the strategic Gallipoli peninsula during the Dardanelles campaign in 1915, but its war turned into a nightmare on the eastern front against Russia.
Tens of thousands of soldiers died in battles that drew in Armenian fighters who fought alongside Russian troops in a bid to cast off Ottoman rule.
An inhuman act
Defeated by Russia in Armenia and the Caucasus, the Ottomans responded by attacking the Armenian minority in their midst.
"There are two alternatives: either the Armenians will liquidate the Turks, or the Turks will liquidate them," an Ottoman official, Mehmed Resid, wrote in his memoirs.
"Faced with the need to choose, I did not hesitate long. Before they do away with us, we will get rid of them."
The arrest and massacre of 2,000 Armenian leaders in Istanbul on April 24, 1915 began what is described as the first genocide of the 20th century -- although modern-day Turkey categorically refutes the term.
In less than a year, hundreds of thousands were forcibly displaced, their possessions seized and many of them killed.
A century on, the mass killings continue to fuel a bitter controversy, regularly upsetting relations between Turkey and the West.
Armenians, backed by many historians and a growing number of foreign parliaments, say up to 1.5 million of their kin were systematically killed in the dying days of the Ottoman Empire.
Turkey admits large scale massacres took place, but says they were perpetrated in self-defence against the Russian threat. Overall it says 500,000 died in fighting and of starvation.
The Armenian academic Rouben Safrastian rejects the Turkish arguments.
"Massacres of Armenians took place well before World War I,", he argues. "The war was simply a good excuse to carry out a criminal plan."
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"For us the question is just as painful as it was 100 years ago," said the vice-president of the Armenian national assembly, Eduard Sharmazanov. "Turkey needs to end its policy of denial and apologise to the Armenian people."
There have been gradual signs of change in Turkey, with Foreign Minister Ahmet Davutoglu last year calling the events of 1915-16 a "mistake" and an "inhuman act" during a trip to the Armenian capital, Yerevan.
"In recent years there have been commemorations in Turkey, university conferences. It's a small revolution," said Turkish analyst Burcu Gultekin Punsmann.
"A pretty deep process of revision is underway in Turkish society, even if it is not yet obvious at the political level."
Conflict in the Middle East
World War I also redrew the map of the entire Middle East, sowing the seeds of the Israeli-Palestinian conflict.
In 1916, Ottoman forces, led by German generals, quickly gained the upper hand over British troops in Palestine and Mesopotamia, an area that covers modern-day Iraq, Kuwait and parts of Syria.
But British forces proved highly adept at mobile warfare in the desert, one of the few places where fighting on horseback was still possible.
They were assisted by the actions of T.E. Lawrence, the fabled British archaeologist who rallied Arab nationalists in revolt against Turkish rule and sultans.
His hit-and-run attacks on Turkish supply lines were a marginal part of the campaign, but the legend of "Lawrence of Arabia" had dramatic propaganda value, and his writings on insurgency tactics remain highly influential.
By 1917, the British had turned the tide of the campaign, taking Baghdad and Jerusalem. By the following year, Allied forces had occupied Damascus and Beirut and had effective control over the entire region.
The Arabs that supported them had bought into promises from Britain and France that they would win independence after the war, but they were to be bitterly disappointed.
Behind the scenes, Britain and France had already carved up the region between themselves under the Sykes-Picot Agreement of May 1916: Libya and Syria to France; Jordan, Palestine and Iraq to the British.
Adding to the confusion, and cutting across their agreements with both the French and the Arabs, the British had also announced the infamous Balfour Doctrine in 1917, in which foreign secretary Arthur Balfour had promised a homeland for Jewish people in Palestine. The doctrine formed the basis for the creation of the Israeli state three decades later, and a conflict that continues to tear apart the region to this day.
The armistice signed at Mudros in Greece on October 30, 1918, marked the final dissolution and dismemberment of the Ottoman Empire. Five centuries of imperial rule was at an end.
But the fighting was not over for Turkey, which spent another four years in a war of reconquest to regain lost lands in Anatolia, particularly against the Greeks. It was these battles that allowed Mustafa Kemal, who would later become Ataturk, to lay the foundations of modern-day Turkey.