The start of World War One is not that simple. It involved a combination of events. The assassination of Archduke Franz Ferdinand was just one of those events.
At the beginning of the 20th century, Serbia was part of the Austria-Hungary empire. Around 1904, Serbia wanted to expand so it decided to break free of it's economic dependence on Austria-Hungary by trading with other countries. At this time, Serbia's major export item was pork and Austria-Hungary was their biggest customer. To punish Serbia, Austria-Hungary created an "economic war" by stopping the import of Serbian pork in 1906. This backfired because Serbia was able to find other customers and increase it's exports. As a result, relations between Austria-Hungary and Serbia began to decrease.
Then in 1908, after a change in foreign ministers, Austria-Hungary tried to annex (take without permission) some Serbian territory. This made Serbia angry even more than they already were.
Then between 1912 and 1913, Serbia was involved in two Balkan Wars. The first Balkan War involved Serbia, Greece, Bulgaria and Montenegro fighting against the Turks of the Ottoman Empire. The Turks were defeated in the first Balkan War. But Serbia, Greece, Bulgaria and Montenegro could not agree on who gets what land they had captured. So the Bulgarians attacked the Serbians and the Greeks. Then the Romanians and Turks attacked the Bulgarians. By the time both Balkan wars had ended, Serbia had added lots of captured territory to it's empire and this made Austria-Hungary even more concerned.
On June 28, 1914, Archduke Franz Ferdinand was assassinated. For some unknown reason, Austria-Hungary believed that Serbia was involved in the killing. Some historians believe that Austria-Hungary was just looking for an excuse to start a war.
On July 19, 1914, Austria-Hungary took advantage of the assassination and delivered a document to Serbia. This document dealt with most of the disagreements between the two countries, not...