The Empire, as we all know, was a very powerful nation and ruled many other countries very sternly. When Dravot and Carnehan took over, they caused the empire to fall because they fought over the throne and over who the power belonged to. You cannot have a joint leadership when only one of them is making all the rulings and calling all the shots without taking into consideration what the other has to say about the ruling. The empire worked as it was before both of them ruled it. However, when they took over, they just caused it to crash horrendously.