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Over the course of the 19th century, rival powers of Europe formed alliances. Germany, Austria-Hungary, and Italy formed the Triple Alliance. Great Britain, France, and Russia formed theTriple Entente. Political instability and competition threatened those alliances. (Italy, for example, eventually entered World War I in opposition to Germany and Austria-Hungary.)
Tensions grew between Austria-Hungary and Serbia as Serbian nationalists attempted to unite all Slavic peoples living in the Balkan region into a single state, including South Slavs of Austria-Hungary.
The assassination of Austrian Archduke Franz Ferdinand and his wife by Bosnian Serb nationalist Gavrilo Princip on June 28, 1914, suddenly stirred up brewing conflicts in the region.
Austria-Hungary used the assassination as an excuse to settle its dispute with Serbia. In anticipation of Russia’s support of Serbia, Austria-Hungary gained support from William II of Germany before presenting a warlike ultimatum to Serbia.
Serbia accepted most of Austria-Hungary’s demands but sought international arbitration on some issues. Convinced that Austria-Hungary was ready for war, Serbia appealed to Russia for support.
Austria-Hungary severed diplomatic relations with Serbia, and, on July 28, 1914, declared war on Serbia. Within a week most of Europe was at war.
As many as 8.5 million soldiers and some 13 million civilians died during World War I.
Chemical weapons, such as mustard gas and phosgene, and new technologies and developments, such as machine guns, tanks, and aerial combat, were introduced. A protocol signed at the1925 Geneva Conference for the Supervision of the International Traffic in Arms restricted the use of chemical and biological agents in warfare.