Why?

In four short years, from 1914 to 1918, over 10 million men and women serving in armed forces on fronts around the world were killed, while double that number were wounded, disabled and disfigured; and at least another 7 million civilians lost their lives as well. Most died horrific deaths. But as time passes by we tend to forget, a century later, how many sacrifices were made day after day on both sides of one the most deadly conflicts in human history. Civilized Productions has produced a wonderful choral album, Sacrifice and Solace, which features an octet called the Toronto Valour Ensemble who sang these carefully selected and uniquely composed songs from that era. It is available on CD Baby. The simple translation of the Arabic word "jihad" is struggle.

The Struggle for Autonomy

The subject of autonomy and self-determination is quite relevant today, as it was leading up to and throughout the Great War, the war to end war. As a reflection of reaping the spoils of war, to the victor went many nations and their respective resources in the 19th Century, absorbed and vertically integrated into each empire in a global game. These empires competed on the battlefield for resources and control of people, and ultimately for markets in order to sell their finished goods.
When the Atlantic Charter was signed in 1941, certain ideal goals (of the Allies' war effort) were stated, which included: no territorial changes made against the wishes of the people and the restoration of self-government to those deprived of it, reduction of trade restrictions and global cooperation to secure better economic and social conditions for all; freedom from fear and want; freedom of the seas; and abandonment of the use of force, as well as the disarmament of aggressor nations.
On January 1, 1942, the Declaration of the United Nations - heralding a world government - was drafted by U.S. President Franklin Roosevelt and British Prime Minister Winston Churchill, and forms the basis of the United Nations as we know it today. The declaration upheld the Wilsonian principles of self determination, and each signatory subscribed to a program of purposes and principles embodied in the Atlantic Charter "essential to defend life, liberty, independence and religious freedom, and to preserve human rights and justice in their own lands as well as in other lands."
The rights of nations to self-determination is a relatively modern concept. In 1914, the concept of 'might over right' was the norm, whereby defenseless nations were simply subjugated and ancient systems of rule and law were replaced by new democratic notions of government, which supported the capitalist views of foreign owners, and ensured people (workers) remained law abiding. However, as a recognized principle of modern international law, self-determination doesn't actually state what constitutes a nation, or which can claim the right to self-determination.
At the outset of WWI, the Austro-Hungarian empire extended across central Europe, and included what are now Austria, Hungary, Slovakia, and the Czech Republic, as well as parts of present-day Poland, Romania, Italy, Slovenia, Croatia, Bosnia and Herzegovina, and the Federal Republic of Yugoslavia. Its post-war dismemberment would result in many future conflicts, as all these states, more precisely their respective leaders, sought autonomy and self-determination for their people, which Western leaders truly could not oppose.
Likewise, the German empire was an offshoot of Prussia, an autocratic nation that had defeated both Austrian and French armies in the century preceding the Great War, led by the powerful statesman Otto Von Bismarck until 1890 yet in fact ruled by a Kaiser, the King of Prussia. When war broke out in 1914, Germany had been a unified state for less than fifty years, with nationalism being fueled by a common language. When Wilhelm II ascended to the throne, he began to expand his empire across Africa, East Asia and The Pacific supporting rapid industrialization.
Dominating the richest regions of Europe, Germany's leaders had hoped to crush the armies of both Russia and France, decisively and quickly in 1914, and continue to achieve their own Kaiser's objective of becoming a truly global superpower.  The Treaty of Brest-Litovsk, negotiated between Germany in 1918 and the Bolshevik government that it had helped to install in Moscow, removed Russia from the war, gave Germany the Baltic states and part of Belarus, and made Ukraine a German satellite.
The German military mission of 1913 to Turkey under Liman von Sanders organized the Turkish army and navy under German leadership and brought forth the Turco-German Alliance. In 1912, the Balkan Wars caused Russia to fear loss of access to the Bosphorus straits. So, a year later, Russia threatened to occupy Ottoman territory if German military under Liman von Sanders was not removed. The Ottoman Empire joined the Central Powers (forming the Triple Alliance) and entered World War I on October 28, 1914, with the bombing of Russia's Black Sea ports.
Prior to the Great War, seeking independence from the Ottomans, in addition to the troubles in Armenia, the Hauran Druze Rebellion erupted in 1909 and was a violent uprising in the Syrian province, in opposition to the spread of taxation, elections and conscription, However, in August 1910, an Ottoman force of some 35 battalions crushed the resistance. In 1911, Muslim intellectuals and politicians formed The Young Arab Society in Paris, whose stated aim was to raise "the level of the Arab nation to the level of modern nations." 
Within years, its leaders called for greater autonomy within a unified Ottoman state rather than Arab independence from the empire. At the Arab Congress of 1913 in Paris, desired reforms were discussed with other dissenting individuals from the Arab world, who also requested that Arab conscripts to the Ottoman army not be required to serve in non-Arab regions except in time of war. However, as the Ottoman authorities cracked down on the organization's activities, leaders went underground and subsequently demanded the complete independence and unity of the Arab provinces.
As well, the Kurds were demanding automony - as early as 1889. Shaykh Abd al Qadir in 1910 appealed for an autonomous Kurdish state in the east. Some leaders urged Kurds to unite and forget their differences, while other Kurdish Shaykhs in the region began leaning towards regional autonomy. During this time, the Badr Khans had been in contact with discontented Shaykhs and chieftains in the far east of Anatolia ranging to the Iranian border to discuss secession.
In 1914, because of this possible Kurdish threat as well as their dealings with Russia, Ottoman troops moved against leaders and the rebellions of Barzan and Bitlis were quickly suppressed. Also, in 1914, General Muhammad Sharif Pasha offered his services to the British in Mesopotamia. Elsewhere, members of the Badr Khan family held close relations with Russian officials and discussed their intentions to form an independent Kurdistan.
In the late 19th century, Yemenis led by Imam Mohammed ibn Yahya rebelled against the Ottomans. When he died in 1904, his successor Imam Yahya ibn Mohammed led the revolt against the Empire in 1904-1905, forcing important concessions, such as restoring sharia law. In 1906, the Idrisi leaders of Asir rebelled against the Ottomans. By 1910 they controlled most of Asir, but were ultimately defeated by Ottoman Modern Army, to maintain their system of government in the Sunni-majority parts of Yemen.  
Italy declared war on the Ottomans on September 29, 1911, launching the Italo-Turkish War, demanding the turnover of Tripoli and Cyrenaica. Italian forces took those areas in November of that year. This oft-overlooked war was an important precursor of World War I, as it sparked nationalism among the Balkan states. The Italians also sent weapons to Montenegro, encouraged Albanian dissidents and seized Rhodes. Members of the Balkan League then attacked the Ottomans before the war with Italy had ended in 1912.  
Following the war, victorious Britain and France carved up the Ottoman empire.