Franz Ferdinand, the Archduke of Austria-Hungary, was assassinated on June 28, 1914
Serbia declined orders from Austria-Hungary. Austria-Hungary wasn't happy and decided to start WW1.
The War began on November 11th, 1918, and Alliances were being made. Overall, Germany, Austria-Hungary, and Italy, were in an alliance against France, Russia, and Britain, were in war against each other. Serbia was in between both sides along with other small countries. Later other countries like the United States and Belgium joined in.
War continued and it became harder and harder to survive