World War One started on June 28,1914 when Nedeljko Čabrinović threw a bomb that eventually killed Austria-Hungary's Archduke Franz Ferdinand in which the goal was for Serbia to gain the south-Slav provinces of Austria-Hungary. It bounced off the back of the Duke's car and injured 16-20 people with a crater with a 1 ft diameter and a 6.5 inch depth. Austria-Hungary rejected Serbia's offer and subsequently went to war.
They both had alliances with other countries globally, Austria-Hungary had an alliance with Prussia (it's "triangular" cross is on the bad airplanes), Germany and the Ottoman Empire and Serbia had Britain, Russia (NOT USSR they dropped out of the war because of the October Revolution), France, and by 1918,America. European colonies provided vital supplies like food, troops and weapons.
The Treaty of Versallies, a peace term that got later nazis to pick Adolph Hitler said that the losing alliance (Austria-Hungary's) had to pay billions of $$, couldn't make a new military, and lost land. That last one is really an opinion, ask a German and he/she'll say it wasn't fair, however, ask a Russian and he/she'll say the Germans deserved it. Me,personally…. I think it part was and part wasn't, I mean even though they started a war, German currency became so low that a wheelbarrow full of cash would get you a loaf of bread during the interwar period. Hoped this helped! ;-D