World War I

Francis went to Sarajevo(Bosnia)
One Bosnian which belong to Black hand(黑手党)killed Francis, the archduke
Then Austra-Hungary want to have the war with Serbia.

Russia, Britain, France and USA support Serbis(Balkin)

Germany support Austra-Hungary

Serbia and Austra-Hungary turned a local war into a general war,so they need to responsible to the broke out of war.

Hope can help you

Serbia was most responsible for ww1 because a Serbian Nationalist named Gavrilo Princip assasinated Austro-Hungarians archduke, Franz Ferdinand.

Austria - Hungary declared war on Serbia .

Serbias allies came to their defence (Russia). Then Austria's allies had no choice but to help them (Germany). Then Germany and Russia's allies came to help them and so on.......

The first World War had begun.