The War of the World Gate started after Guthix opened the World Gate for the first time and allowed several powerful races to come through it and start a civilization. Many of them were confused, and the different races started to fight each other in a bloody battle which eventually led to Guthix wiping out all the combatants and closing the World Gate, realizing his mistake. It wasn't until 1000 years later that Guthix decided to open the World Gate again, this time with more success.