The Assassination of Archduke Ferdinand of Austria and his wife Sophie.
While tensions between the Central Power (Austrio-Hungary, Ottoman Empire, Germany, and their colonies) and the Allied Powers (Almost the rest of the world, including Italy, which left the Central Powers) was extremely high because of the events surrounding the annexation of lands that both Serbia and Austrio-Hungary wanted. This led to the Assassination of the said couple, and with this, Austrio-Hungary found a reason to both declare war and try to take over Serbia. However, the Allies would not have any of this, and soon it set off a chain-reaction of declaration of wars, leading to the first, truly global war.
~