Answer :

June 28, 1914 ^-^ hope that helped
1914, after the duche of Austria is shot dead in Serbia. That ignited 1st world war, but did not cause world war 1. 

The main cause of world war 1 is for secret alliance formed between nations. When Austria declare war on Serbia, Russia comes in to side with Serbia, then Germany joins Austria, and so on. It became a big mess that was never intended that way.