The event that set off World War I was the assassination of Archduke Franz Ferdinand of the Austro-Hungarian Empire and his wife Sophie by Gavrilo Princip, a Serbian nationalist, on June 28, 1914 in Sarajevo. The assassination resulted in the July Crisis, during which Germany, Austria-Hungary, France, Britain and Russia entered into frantic diplomatic maneuvering in an effort to prevent the outbreak of war. The negotiations proved unsuccessful, and on July 28, Austria-Hungary declared war on Serbia, an act which quickly led to an all-out conflict between all of the involved nations.
Because of the entangled alliances and longstanding rivalries between the many nations involved, the war developed quickly into a worldwide conflict incurring millions of military and civilian casualties. The actual fighting began when troops from Austria-Hungary invaded Serbia on August 12, but soon extended across the European continent and into the Middle East. The German advance against France was halted, but by the end of 1914, it became the line of trenches collectively called the Western Front. This battle line remained virtually unchanged until 1917, and it grew into a grinding and stalemated war of attrition with casualties estimated in the hundreds of thousands in the course of a single engagement.
Historians are not in complete agreement regarding the issue of whether the war might have been avoided if the Archduke's assassination had not occurred. Many believe that the "European powder keg" would have eventually been ignited by some other spark owing to the many tensions and rivalries existing between the major powers in the years leading up to 1914.