Why did World War I turn into a global war as opposed to just a regional conflict? The answer lies in the alliance system. The alliance system was in place far before World War I started but a series of events caused the alliances to take a more prominent form and consequently have a larger role in the start of the Great War. There were multiple reasons behind that that contributed to the alliance system being the main cause of the war. The assassination of Franz Ferdinand, the archduke of Austro-Hungary, tied into as one of the main instigators for joining the countries together into more prevalent relations; the Dual Alliance was formed, and later the Central Alliance; and nationalism played a key role in the coming together of the alliances, as it promoted a sense of unity for their own countries and in turn led them to fight against the countries on the opposite alliance party. All three of those reasons contributed to alliances being the main cause of World War I, as opposed to any other cause.
The assassination of Archduke Franz Ferdinand prompted many different European nations to coalesce and form a clearer line for who was defending whom and who was attacking whom. The war really started with Serbian nationalist Gavrilo Princep assassinating the archduke in Sarajevo, Bosnia, in 1914 which was a part of Austro-Hungary in protest of Austro-Hungary having control of Bosnia. Serbia wanted to take over Bosnia and Austro-Hungary declared war on Serbia as a result of this. Consequently, Russia began to mobilize due to their alliance with Serbia, so Germany, as an ally of Austro-Hungary, declared war on Russia. The assassination triggered the respective nations to arise and establish their sides. Thus began the expansion of war to include all those involved in the mutual defense alliances. .
In 1879, the Dual Alliance treaty was formed. It comprised of Germany and Austro-Hungary, who signed to protect each other if Russia, a common enemy, attacked.
