From what I read in some history books, the assassination of Franz Ferdinand was just one of many pieces that led to WWI.
The way that I often hear it described is the spark that blew up the powder keg that was Europe at the time. Obviously a spark alone would do very little without the gunpowder there to be ignited, so there’re definitely different levels of causation, but I think there is a sense in which the assassination can be seen as a causative element
It was an action which was taken as a reason to start a war, so yes, there was causation