What is the ruling if the Imam is unable to subdue both fighting rebel groups and fears they might unite against him?

Primary text

If the Imam is unable to subdue both groups and fears their unification in war against him, he joins the one nearest to the truth. If they are equal, he uses his own ijtihad (legal reasoning) in joining one of them. This is not done to aid one party specifically, but rather to seek assistance against the other. This is the position of Al-Shafi'i.