World War One, otherwise known as the "Great War- lasted from 1914 to 1918. There are many opinions as to what started the war, but I believe it was not just one event, but a chain of events that started the war. So in the following paragraphs I will give a brief summary describing the events that led to the Great War.
The spark of the war was the assassination of Archduke Franz Ferdinand, heir to the Austro-Hungarian throne. He was assassinated in Sarajevo on June 28th, 1914. Ferdinand's death was at the hands of the Black Hand, a Serbian terrorist society, and these series of events are what sparked the world's first global war. Arguing that the Serbian government was responsible for the actions of the Black Hand, the Austro-Hungarians took the opportunity to stamp its authority upon the Serbians, crushing the nationalist movement there and strengthening Austria-Hungary's influence in the Balkans. It did so by issuing an ultimatum to Serbia, which in the list of its demands, the assassins are brought to justice. Sir Edward Grey, the British Foreign Secretary, was impressed, and commented that he had never seen one State address to another independent State a document of such demands. Austria-Hungary's expectation was that Serbia would reject the ultimatum, thereby giving them a reason for launching a declaration of war against Serbia. However, Serbia had long had Slavic ties with Russia. And Russia felt obligated to help their Slavic Brothers. The Austro-Hungarian government expected assistance from their ally, Germany, which they would come to their aid if the unthinkable happened and Russia declared war on Austria-Hungary. Germany quickly agreed, and even encouraged Austria-Hungary's warlike attitude. So then, we have the following sequence of events that led to the 'Great War' - a name that had been given to it before it had even started.
Austria-Hungary, unsatisfied with Serbia's response to their ultimatum, declared war on Serbia on July 28th, 1914.