There are many factors that led to the outbreak of WWI and they all have their importance in history of WWI, but there are three factors that I think are quite important.
1. The Ultimatum sent to Serbia from Austria-Hungary.
On July 23, 1914, Austria-Hungary sent an Ultimatum to Serbia with 10 really tough demands; they gave Serbia only two days to revise and expected Serbia to refuse the Ultimatum completely. But the Serbs agreed most of the Ultimatum politely, but they did not agree with the 6th demand, which was to send Austria-Hungary polices into Serbia and arrest all the terrorists that was in the "Black Hand" ("Black Hand" was the terrorist association that planned the assassin of Archduke Franz Ferdinand). With this excuse, Austria-Hungary declared war against Serbia 3 days after receiving the refuse.
2. Russia's action of preparing army.
On July 31, 1914, Russia was preparing their army for battle, which affected Austria-Hungary because Serbia was Russia's ally, and Russia swore to never let down Serbia again after the Bosnian Crisis in 1908.
But the leader of Russia Tsar Nicholas was the cousin of Kaiser Wilhelm, leader of Germany, so he sent a letter to the Kaiser saying that his action of preparing army was not for the use of attacking Germany. But Kaiser used this action as an excuse and declared war against Russia on August 1, 1914.
3. The Schlieffen plan and the treaty between U.K and Belgium.
Germany was surrounded by enemies during that time in Europe, and their 2 biggest opponents Russia and France were on two different sides, attacking any of them with full strength would cause the other one to take over Germany from the other side, so Germany had to use their only plan- The Schlieffen plan. The plan was...