Austria-Hungary is the main country that started World War I. Although it is common knowledge that the war started due to outrage over the assassination of Arch-Duke Ferdinand, Emporer Franz Josef expressed his relief at the death of Ferdinand. Austria-Hungary had been looking for a reason to wage a war with neighboring Serbia, and with the assassination of Ferdinand, they finally had a reason to attack. Austria-Hungary had the backing of Germany, and possibly Russia, when they finally had a reason to truly attack Serbia. They did not expect Russia to enter the mix, and assumed that it would be a small war.
Austria-Hungary gave Serbia an ultimatium which they assumed they would reject, but came back with accepting most of the terms of the agreement. As a result, Russia, who had a treaty with Serbia, came to thier aid; Germany had a treaty with Austria-Hungary and came to their aid; France had a treaty with Russia was now at war with Germany by alliance, and invades Belgium to have an easy route to France; and England had a treaty with France, is not at war with Germany. All of these things combined show that Austria-Hungary started what eventually became known as the Great War.