I don't have a lot of time to explain this, but there were a lot of different things that led to the cause of WW1. One of the main reasons that WW1 happened was because of the assassination of the Archduke Franz Ferdinand of Austria by a Serbian Nationalist. To avoid conflict, Austria gave a list of demands to Serbia. Unfortunately, Serbia was unable to meet these demands, and Austria declared war on Serbia. Essentially, this was the catalyst of the war, and because of all of the alliances, multiple countries were declaring war on each other.