How did the alliance system turn a dispute between two countries into world war i?

Although the imperialism that had been developing for decades the powers involved was the main underlying cause, the trigger for the conflict occurred on June 28, 1914 in Sarajevo with the assassination of Archduke Franz Ferdinand of Austria, at the hands of Gavrilo Princip, a young man Serbian nationalist. This event sparked a diplomatic crisis when Austria-Hungary gave an ultimatum to the Kingdom of Serbia and invoked the various international alliances forged over the previous decades. In a few weeks, all the great European powers were at war and the conflict spread to many other geographical areas.

1. i forgot 2. he was gay 3. they killed a lot of the population 6. they rode horses and had swords we have guns and tanks