On July 28, 1914, ArchDuke Franz Ferdinand II was assassinated in Sarajevo. This provoked perfect opportunity for Austria-Hungary to gain public support for a war against the Serbs. The assassination is probably the most important event which led to WWI. After Austria-Hungary went to war on Serbia, Russia declared to stand by the Serbs. This made Serbia less willing to agree to Austro-Hungarian demands that might otherwise have been the case.The tzar escalated the crisis by mobilising his troops against Austria-Hungary. This gave perfect opportunity for German goverment to enter the crisis as a defender of it's alliance partner. Russia's determination not to back down against German aggresion was also largely due to the widely held fears that Germany wanted to gain territory at Russia's expence.
Russia's joining forces with Serbia brought in Germany in the crisis. The Germans supported Austria-Hungary's going to war with Serbia. Germany used the minor Hapsburg-Serb quarrel as a pretext to launch it's own big escape from diplomatic encirclement by launching a European war.
The importance of the alliance system must also be considered in explaining the outbreak of the war.Germany was determined to support it's Triple Alliance partner during the crisis, but followed it's own aims. The Russians were determined to support Serbia, the French were determined to support Russia, Thus bringing in almost all Europe into the war. Although, each power went to war not because of alliance commitments, but by deciding what was best for it's own national interests. It must be remembered that Britain had no firm alliance with France or Russia, but went to war all the same.
Between all these points, I think the most important event that led to the outbreak of the war was in fact the alliance system, and each major power's...