What is a major cause of wwi? /the last straw?

The last straw that caused that triggered the outbreak of WWI was the assassination of Archduke of Austria - Hungary Ferdinand and his wife on June 28, 1914 by a Serbian nationalist with ties to the secretive military group known as the Black Hand.

Due to this, one month later, Austria - Hungary declared war on Serbia. The situation was joined by other countries based on which alliance they were in. A group of alliances between major powers in Europe had been formed or were in the process of being formed by this time.

its true: ) hope this