Austria-Hungary's ultimatum to Serbia: In response to the assassination, Austria-Hungary issued an ultimatum to Serbia that was deliberately designed to be unacceptable. The ultimatum included demands that Serbia suppress all anti-Austrian propaganda, punish those responsible for the assassination, and allow Austrian officials to participate in the investigation of the crime. Serbia accepted most of the demands, but rejected the one that would have allowed Austrian officials to participate in the investigation.
Austria-Hungary's declaration of war on Serbia: On July 28, 1914, Austria-Hungary declared war on Serbia after the Serbian government refused to accept all of the demands in its ultimatum.
Russia's mobilization: In response to Austria-Hungary's declaration of war on Serbia, Russia began to mobilize its army. Germany, which was allied with Austria-Hungary, viewed Russia's mobilization as a threat and issued an ultimatum to Russia demanding that it cease mobilization. Russia refused to comply with the ultimatum, and Germany declared war on Russia on August 1, 1914.
France's and Britain's declarations of war on Germany: France, which was allied with Russia, declared war on Germany on August 3, 1914. Britain, which was allied with France and Belgium, declared war on Germany on August 4, 1914.
The widening of the conflict: The war initially involved Austria-Hungary, Serbia, Russia, Germany, France, and Britain. As the war continued, other countries entered the conflict, including Italy, Japan, the United States, and the Ottoman Empire. By 1918, more than 30 countries were involved in the war.
The global impact of the war: World War I had a profound impact on the world. It resulted in the deaths of millions of people, the destruction of cities and towns, and the displacement of millions of people. The war also led to the collapse of the Russian Empire, the Austro-Hungarian Empire, and the Ottoman Empire. It also led to the rise of the United States as a global power.