The First World War broke out in 1914.
By the late 19th century, major European nations scrambled for colonies. Conflicts occurred. European nations engaged themselves in an arms race, which made war more likely.
By 1907, Europe was divided into two rival camps, the Triple Alliance and the Triple Entente. Any quarrel between members of these two camps would easily lead to a world war.
The nature of nationalism became extreme in the late 19th century. To promote their self-interests, European powers would even go to war with others.
The assassination of Archduke Ferdinand started the First World War.