What was the factor that contributed to the out break of world war 1?

it was people smoking in a powder keg room, an assassination was the spark.

There were many factors and attacks but in spite of all these confrontations, it was an attack that lit the flame that blew everything up and war was declared. It was the assassination of Archduke Franz Ferdinand, heir to the throne of the Austro-Hungarian Empire on 28 June 1914 in Sarajevo.