Although the spark that started World War I was the assassination of Archduke Franz Ferdinand, the heir to the Austro-Hungarian throne, carried out by Gavrilo Princip, a Bosnian-Serbian nationalist, three causes created the right political and military circumstances for this global armed conflict to take place: 1) the geopolitical, economic, and military rivalry between Great Britain and the German Empire; 2) the system of alliances that forced each of the European powers to get involved in the war; 3) the late 19th and early 20th century arms race, known as the Armed Peace. In 1907, Great Britain, France, and Russia had signed the Triple Entente, which was a military defensive pact to counteract the Triple Alliance, which was a military alliance that had been signed, in 1882, by Germany, Austria-Hungary, and Italy.
Upon the assassination of Franz Ferdinand and his wife in Sarajevo, on June 28, 1914, Austria-Hungary sent Serbia an ultimatum on July 23, demanding the Serbian government to dissolve the Serbian nationalist organization Narodna Odbrana, held responsible for plotting the assassination, and to condemn and suspend any publications that provoked hatred of the Austro-Hungarian monarchy. Sarajevo was the capital of Bosnia and Herzegovina, which had been annexed from Serbia to the Austro-Hungarian Empire in 1878. Since the Slav country did not answer satisfactorily the ultimatum, the Austrian emperor, Francis Joseph I, declared the war on Serbia on July 28. Because Russia was bound to Serbia a military alliance, the Czar Nicholas II ordered a mobilization against Austria-Hungary. As a result, the German Empire declared the war on Russia on August 1. Since France threatened Germany with a military mobilization and redeployment of troops, Germany declared the war on the French on August 3 and proceeded to attack Belgium in order to invade France. Faithful to the Triple Entente agreements, Great Britain declared war on Germany on August 4, 1914. Turkey sided with Germany due to a previous military pact, while Italy, which had decided not to abide by the Triple Alliance agreements, declared war on Austro-Hungary. Thus, WW1 was a three-front armed conflict, with a Western, Eastern, and a Southern Front.
During the first months, the war was dynamic as Germany implemented the Schlieffen Plan, moving his forces across Belgium to surround and defeat the French Army in northern France. However, the British and French forces managed to stop the German troops at the First Battle of the Marne on September 12, 1914. From then on, the war would become a static armed struggle, with both sides launching ineffectual attacks and counterattacks from long lines of trenches, in which the soldiers of both sides found protection from artillery and machine gun fire, as they tried to conquer enemy-held ground by attacking the opposite line of trenches in suicidal attacks. This was how World War I was characterized.