On July 28, 1914, one month to the day after Archduke Franz Ferdinand of Austria and his wife were killed by a Serbian nationalist in Sarajevo, Austria-Hungary declares war on Serbia, effectively beginning the First World War.

Threatened by Serbian ambition in the tumultuous Balkans region of Europe, Austria-Hungary determined that the proper response to the assassinations was to prepare for a possible military invasion of Serbia. After securing the unconditional support of its powerful ally, Germany, Austria-Hungary presented Serbia with a rigid ultimatum on July 23, 1914, demanding, among other things, that all anti-Austrian propaganda within Serbia be suppressed, and that Austria-Hungary be allowed to conduct its own investigation into the archduke’s killing. Though Serbia effectively accepted all of Austria’s demands except for one, the Austrian government broke diplomatic relations with the other country on July 25 and went ahead with military preparedness measures. Meanwhile, alerted to the impending crisis, Russia—Serbia’s own mighty supporter in the Balkans—began its own initial steps towards military mobilization against Austria.

In the days following the Austrian break in relations with Serbia, the rest of Europe, including Russia’s allies, Britain and France, looked on with trepidation, fearing the imminent outbreak of a Balkans conflict that, if entered into by Russia, threatened to explode into a general European war. The British Foreign Office lobbied its counterparts in Berlin, Paris and Rome with the idea of an international convention aimed at moderating the conflict; the German government, however, was set against this notion, and advised Vienna to go ahead with its plans.

On July 28, 1914, after a decision reached conclusively the day before in response to pressure from Germany for quick action—apart from Kaiser Wilhelm II, who by some accounts still saw the possibility of a peaceful diplomatic resolution to the conflict, but was outmaneuvered by the more hawkish military and governmental leadership of Germany—Austria-Hungary declared war on Serbia. In response, Russia formally ordered mobilization in the four military districts facing Galicia, its common front with the Austro-Hungarian Empire. That night, Austrian artillery divisions initiated a brief, ineffectual bombardment of Belgrade across the Danube River.

“My darling one and beautiful, everything tends towards catastrophe and collapse,” British naval official Winston Churchill wrote to his wife at midnight on July 29. He was proven right over the next several days. On August 1, after its demands for Russia to halt mobilization met with defiance, Germany declared war on Russia. Russia’s ally, France, ordered its own general mobilization that same day, and on August 3, France and Germany declared war on each other. The German army’s planned invasion of neutral Belgium, announced on August 4, prompted Britain to declare war on Germany. Thus, in the summer of 1914, the major powers in the Western world—with the exception of the United States and Italy, both of which declared their neutrality, at least for the time being—flung themselves headlong into the First World War.