Rather easy Hungary wanted more land (so did Germany) and they where backed by Germany. Hungary attack Serbia which was beginning to militarize and was also backed by Russia(soon to be USSR) and thus they started fighting to defend/support their allies. France had also allied with Russia then France declared war on Germany, the Germans had a 2 front war to fight. France was also backed by Britain another world class super power at the time. and thus began the first world war. I got a bit off topic there but I hope this helps!
