What was one of the causes of tentions in Europe that led to world war 1

The real causes of World War I included politics, secret alliances, imperialism, and nationalistic pride. However, there was one single event, the assassination of Archduke Ferdinand of Austria, which started a chain of events leading to war.

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.