The July Crisis was a series of interrelated diplomatic and military escalations among the major powers of Europe in the summer of 1914, which led to the outbreak of World War I (1914–1918). The crisis began on 28 June 1914, when Gavrilo Princip, a Bosnian Serb, assassinated Archduke Franz Ferdinand, heir presumptive to the Austro-Hungarian throne, and his wife Sopie Duchess von Hohenberg. The assassin team was armed, trained, smuggled across the border and instructed by the head of the Serbian military intelligence service and his staff. A complex web of alliances, coupled with miscalculations when many leaders regarded war as in their best interests or felt that a general war would not occur, resulted in a general outbreak of hostilities among most major European nations in early August 1914.

Austria-Hungary viewed the irredentist movements of South Slavs, as promoted by Serbia, as a threat to the unity of its multi-national empire. Following the assassination, Austria sought to inflict a military blow on Serbia to demonstrate its own strength and to dampen Serbian support for Yugoslav nationalism. However, Vienna, wary of the reaction of the Russian Empire (a major supporter of Serbia), sought a guarantee from its ally Germany that Berlin would support Austria in any conflict. Germany guaranteed its support, but urged Austria to attack quickly, while world sympathy for Ferdinand was high, in order to localize the war and to avoid drawing in Russia. Some German leaders believed that growing Russian economic power would change the balance of power between the two nations, that a war was inevitable, and that Germany would be better off if a war happened soon. However, rather than launching a quick attack with available military forces, Austrian leaders deliberated into mid-July before deciding that Austria would give Serbia a harsh ultimatum on 23 July and would not attack without a full mobilisation of the Austro-Hungarian Army (which could not be accomplished before 25 July 1914).

Just prior to the Serbian reply to the ultimatum, Russia decided that it would intervene in any Austro–Serbian war and ordered a secret but noticed partial mobilization (Maskirovka) of its armed forces. While Russian military leadership acknowledged that Russia was not yet strong enough for a general war, Russia believed that the Austrian grievance against Serbia was a pretext orchestrated by Germany and that Saint Petersburg needed to show strength in support of its Serbian client. The Russian partial mobilization – the first major military action not undertaken by a direct participant in the conflict between Austria-Hungary and Serbia – increased the willingness of Serbia to defy the threat of an Austrian attack and greatly increased the alarm in Germany about masses of Russian troops assembling near its borders. Previously, the German General Staff had predicted that Russian mobilization in the east would be slower than that of Russia's French ally on Germany's western border; therefore, German military strategy in any conflict with Russia involved attacking France through Belgium (to avoid French fixed defenses) and quickly defeating France in the west before turning to face Russia in the east. France, aware that it would have to act together with its Russian ally to defeat its German rival, escalated its military preparations as tensions along the Russian border increased, which, in turn, further alarmed Germany.

While the United Kingdom was semi-formally aligned with Russia and France, it also had relatively friendly diplomatic relations with Germany, and many British leaders saw no compelling reason to involve Britain in a continental war. Britain repeatedly offered to mediate, using the Serbian reply as the basis of negotiation, and Germany made various promises in an attempt to ensure British neutrality. However, Britain decided that it had a moral obligation to defend Belgium and to aid its formal allies, and thus became the last major country actively involved in the July Crisis to formally enter the conflict on 4 August. By early August, the ostensible reason for armed conflict, the dispute between Serbia and Austria-Hungary over the murdered heir, had already become a sidenote to a general European war.