Build-up of military alliances is the major cause of the "Great War;" or what is known commonly today as World War I or the First World War. These alliances developed in the late 19th Century (or late 1800's). this is due to the need for smaller, less prowess nations to seek backing from stronger nations to prevent the spead of Eurpoean colonial take-over.
To place a specific date on the start of the conflict one would reference the assassination of the Archduke of the Austro-Hungarian Empire, named Franz Ferdinand, on 28 June 1914 in the city of Sarajevo by a member the of the Black Hand, a pro-Serbian terrorist organization, named Gavrilo Princip. The nation of Serbia wanted its independence from the Austro-Hungarian Empire that claimed rule over it. Serbia, a smaller nation was militarilly allied with the Triple Entente (Russia, Great Britain and France) and when the Austro-Hungarian Empire declared war on Serbia in retailiation to assassination Russia came to it's aid. Then a "domino-effect" of war declarations occurred. The Great War lasted until Germany was finally defeated November 11, 1918. The Treaty of Versailles officially ended the conflict when it was presented to the German delegation for a signature on May 7, 1919.
In short: World War I lasted from 28 June 1914 until 7 May 1919. However, it's important to note that there is NO World War I without a World War II. The treaty that ended the Great War or World War I is what caused the rise of the NAZI Party and it's leader Hitler in Germany years later that is responsible for the starting World War II . This is due to the harshness it placed on the German nation in the form of reparation payments, military disarmament and the dissection of its nations land.