Austria-Hungary Declares War on Belgium
At the outbreak of the First World War, the German army (consisting in the West of Seven Field Armies) executed a modified version of the Schlieffen Plan, designed to quickly attack France through neutral Belgium before turning southwards to encircle the French army on the German border. The plan called for the right flank of the German advance to converge on Paris and initially, the Germans were very successful, particularly in the Battle of the Frontiers (14 August–24 August). By 12 September, the French with assistance from the British forces halted the German advance east of Paris at the First Battle of the Marne (5 September–12 September). The last days of this battle signified the end of mobile warfare in the west.
On 28 June 1914, Gavrilo Princip, a Bosnian-Serb student and member of Young Bosnia, assassinated the heir to the Austro-Hungarian throne, Archduke Franz Ferdinand of Austria in Sarajevo, Bosnia. This began a period of diplomatic manoeuvering between Austria-Hungary, Germany, Russia, France and Britain called the July Crisis. Wanting to end Serbian interference in Bosnia conclusively, Austria–Hungary delivered the July Ultimatum to Serbia, a series of ten demands which were deliberately unacceptable, made with the intention of deliberately initiating a war with Serbia. When Serbia acceded to only eight of the ten demands levied against it in the ultimatum, Austria–Hungary declared war on Serbia on 28 July 1914. Strachan argues "Whether an equivocal and early response by Serbia would have made any difference to Austria-Hungary's behaviour must be doubtful. Franz Ferdinand was not the sort of personality who commanded popularity, and his demise did not cast the empire into deepest mourning".
Wikipedia: World War I
Wikipedia: Western Front (World War I)