Answer:
Look at Explanation.
Step-by-step explanation:
Archduke Franz Ferdinand of Austria made a trip to Serbia where he was killed by a Serbian/Bosnian nationalist group. Because of this Austria had several demands for Serbia/Bosnia. Serbia met most of them but some of them were outrageous. In response Austria declared war on Serbia. Germany sided with Austria, and Russia with Serbia. Russia then mobilized their military and in response Germany did as well.
This then cause many more nations to enter the war because Russia was part of the Entente powers which included Russia, France, Italy, and Britain. Germany on the other hand was part of the Central powers which included Germany, Austria-Hungary, the Ottoman Empire, and Bulgaria.
Hope this helps.