The countries and regions that took part in World War I (WWI) and their reasons are as follows:

- Austria-Hungary: because their heir to the Austro-Hungarian throne, Archduke Franz Ferdinand, was assassinated by a Bosnian Serb named Gavrilo Princip.
 
- Serbia: because Austria-Hungary declared war against them.

- Russia: because they wanted to defend Serbia.

- Germany, France, and Britain: because they were allies with the countries already engaging in the war.

- Middle East, the Alps, the Balkans, and overseas colonies, bringing Bulgaria, Romania, Greece, and others: because the Eastern Front of the war was more fluid, but neither side could gain a decisive advantage. Both sides had attempts to overcome the stalemate leading to the war expanding into these regions and countries.