Austria-Hungary, Germany and Russia

A handful of bellicose political and military decision-makers in Austria-Hungary, Germany and Russia caused WW1.

Relatively common before 1914, assassinations of royal figures did not normally result in war. But Austria-Hungary's military hawks - principal culprits for the conflict - saw the Sarajevo assassination of the Austro-Hungarian Archduke Franz Ferdinand and his wife by a Bosnian Serb as an excuse to conquer and destroy Serbia, an unstable neighbour which sought to expand beyond its borders into Austro-Hungarian territories. Serbia, exhausted by the two Balkan wars of 1912-13 in which it had played a major role, did not want war in 1914.

Broader European war ensued because German political and military figures egged on Austria-Hungary, Germany's ally, to attack Serbia. This alarmed Russia, Serbia's supporter, which put its armies on a war footing before all options for peace had been fully exhausted.

This frightened Germany into pre-emptively declaring war on Russia and on Russia's ally France and launching a brutal invasion, partly via Belgium, thereby bringing in Britain, a defender of Belgian neutrality and supporter of France.

https://www.bbc.com/news/magazine-26048324