First, you need to go to World Laws.
Then, you need to turn off diplomacy for this to work

Now that Diplomacy is turned off, you need to give the kingdoms 'spite' to start the never ending war
If you want to end the war, give the kingdoms 'friendliness' and they will stop fighting

-The war will not end until one of the countries is conquered or destroyed
-Since Diplomacy is turned off, rebellions cant happen, so if you want rebellions you need to give the targeted kingdom/s 'Inspiration'

That's basically it its really simple and easy to make a never-ending war. It can help if you constantly have to start wars again because the kingdoms make peace.