What were the causes of WW1

Biggest one is the assassination of Franz Ferdinand.

Before, there was the Triple Alliance (Austria-Hungary, Germany, and Italy) and the Triple Entente (France, Great Britain, and Russia).

It started when Austria-Hungary assassinated Archduke Francis Ferdinand and Serbia decided that Austria-Hungary was now its enemy. Events took a turn from the worse when allies got in on the action. Since Austria-Hungary and Germany had taken the offensive, Italy broke away. The Triple Alliance became the Central Powers (Austria-Hungary, Germany, and Ottomans). The Triple Entente became the Alliances (Great Britain, France, England, Italy, etc.). Tensions, especially with Germany, rose and wars broke out. At the time, the British had the largest empire in the world and used its colonies to its advantage, consequently making it a world war.