Answer:
Austria Hungary and Serbias hate for each other.
Step-by-step explanation:
Austria-Hungary annexed allies of Serbia. The Archduke visited the city on a nationalist holiday and was assasainated by a man named Gavarillo Princip. Germany sided with Austria-Hungary as it declared war on Serbia. Russia, France, Britain decis\ded to side with each other as a whole. The assssination threw off everyone because people were expecting war but did not really know when it would come. The assassination just pushed everyone to war.