How do you start a war with an enemy that is already at war with themselves?

The two opposing nations of Serendia and Ibunda - the former a typical monarchical nation and the latter a tribal confederation of nomads. Serendia has been traditionally held to be a just nation, while Ibunda has been seen as a tribal nation that invaded and occupied lands that were not their own.<br><br>Historically, Ibunda has been a splinter nation born out of the remnants of an empire that was conquered by Serendia almost a century ago. The nomads that make up Ibunda have been at odds with Serendia for almost a century and vice versa.<br><br>The primary reason for the animosity between Ibunda and Serendia has been due to the fact that Ibunda has been repeatedly looted and pillaged by the same nomads that make up Ibunda, which has led to a permanent division between the two nations and a mutual hatred between them.<br><br>The answer to the question is simply to create a casus belli, which is any action taken or event that occurs which is used to justify a war to the people and the nation invaded. In this case, the casus belli (or justification for war) is one of military conquest as Ibunda is allegedly seen as a threat to Serendia's territorial integrity.<br><br>However, it is not that simple, as Ibunda is made up of tribal units with different degrees of autonomy, and an invasion cannot be started by just declaring war or sending an ultimatum without properly preparing. So, how do you start a war with an enemy that is already at war with itself?<br><br>Assuming that the war with Ibunda is just, how would you start the war with Ibunda? How would you move to start this war, knowing that Ibunda has been at war with itself for almost a century?<br><br>**TL;DR: How do you start a war with an enemy that is already at war with itself?**