On June 28, 1914, Bosnia's Archduke Franz Ferdinand was assassinated by Gavrilo Princip, a member of "Black Hand," a nationalist group in Serbia. Ferdinand was the heir to the Austrian throne, and his uncle Franz Joseph was forced by the government in Vienna to issue an ultimatum to the Serbians because of the incident. The Serbians' failure to meet the demands started the war between Austria and Serbia, which immediately escalated when Russia declared war against Germany after Germany decided to support Austria.
Although Ferdinand's assassination is considered the spark of the Great War, there were underlying economic reasons as to why the war occurred. The countries that made up the Allied Powers and the Central Powers were already trying to expand and conquer additional territories. France had great reason to support Russia in the war, not only because it condemned the assassination that occurred, but because Germany was trying to expand its territory in Africa, a place that was already dominated by France and Great Britain.
After the Franco-Prussian War, Germany continued to build its military force to prepare to defend itself against France. France, on the other hand, also saw the need to build its military defenses to thwart off any imperialist attack from other countries. In addition, Germany was a threat to the British because of its large naval force. Thus, the Great War gave France and Britain the chance to join forces against a common threat to both nations.
Germany became a unified nation in 1871. Thus, the Germans felt a high regard for keeping their pride as a nation. The French, on the other hand, felt the need to redeem their pride after they lost to Germany in a previous war. Thus, the Great War may have started off because of the rift between Austria and Serbia, but it became a full-blown war because of previous sentiments that the entangled alliances had against each other.