Video: '28th July 1914: Austria-Hungary declares war on Serbia'

(Tuesday, July 28, 1914, 11:00 a.m. Central European Time; during World War I) — One month to the day after Archduke Franz Ferdinand of Austria and his wife were assassinated by a Serbian nationalist in Sarajevo, Austria-Hungary declared war on Serbia today by telegram, effectively launching the First World War.

Threatened by Serbian ambition in the tumultuous Balkans region of Europe, Austria-Hungary decided that the proper response to last month’s assassinations was to prepare for a possible military invasion of Serbia.

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After securing the unconditional support of its powerful ally, Germany, Austria-Hungary presented Serbia with a rigid ultimatum on July 23, demanding, among other things, the suppression of anti-Austrian propaganda and allowing Austria-Hungary to conduct its own investigation into the archduke’s killing.

Though Serbia accepted all of Austria’s demands except for one, Austria-Hungary broke diplomatic relations with Serbia on July 25 and initiated military preparedness measures and declared war today.

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Meanwhile, Russia mobilized in Serbia’s defense, and by Aug. 4, Germany, Russia, France, and the United Kingdom were drawn into the war, with the Ottomans joining in November of the same year.

As the fighting expanded to more fronts, Italy, Bulgaria, Romania, Greece and others joined in from 1915 onward.

In April 1917, the United States entered the war on the Allied side following Germany’s resumption of unrestricted submarine warfare against Atlantic shipping.

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Fighting would take place mainly in Europe and the Middle East, as well as parts of Africa and the Asia-Pacific, and was characterized by trench warfare and the use of artillery, machine guns, and chemical weapons (gas).

World War I would become one of the deadliest conflicts in history, resulting in an estimated 9 million military dead and 23 million wounded, plus up to 8 million civilian deaths from causes including genocide.

The movement of large numbers of troops and civilians was also a major factor in spreading the Spanish flu pandemic.