What cause world war 1 to begin?

Archduke Ferdinand Archduke of Austria-Hungary was assassinated in Sarajevo by the Serbian nationalist Gavrilo Princip. This assassination led to a series of interlocking alliances that led to WWI.

Explanation: World War I was the one of the greatest and most horrific conflicts in human history.

The immediate cause of World War I that made the aforementioned items come into play (alliances, imperialism, militarism, nationalism) was the assassination of Archduke Franz Ferdinand of Austria-Hungary. When Russia began to mobilize due to its alliance with Serbia, Germany declared war on Russia.