Answer:
Because of the Archduke of Austria-Hungary, Franz Ferdinand's assassination.
Explanation:
When taking a drive in an open top car through Serbia with the routs published ahead of time, was assassinated(hard to believe right?). But Austria-Hungary thought that the Serbian government was involved, which they may have been. Austria-Hungary went to their ally, The German Empire. They asked if it was okay to go to war, and Germany was all for it. Austria-Hungary then sent a list of impossible demands to Serbia. Once they refused, Austria-Hungary declared war on Serbia. As France and the United Kingdom were allied with Serbia, they declared war on Austria. Since Germany was allied with Austria, they declared war on France and the United Kingdom. Also, Russia was allied with Serbia, and declared war on Austria. Germany then declared war on Russia.
I hope this helps you and if it did mark as brainiest :)
