Answer :
the complex alliance systems that were in place at the time. Hungary declared war on Serbia. Russia had to back their ally up, and did Germany. (Russia and Serbia were allies). Then Geramny took the opportunity to finally invade france, and G.B. declared war on germany. the ottoman empire were also allied with germany and austrio hungary.
The assassination of Archduke Ferdinand from Austria-Hungary by Serbian's. He was assassinated because Serbia wanted more land (Imperialism)