三十六计 Thirty-Six Stratagems: 隔岸观火 Watch the fires burning across the river
Chapter 2: Enemy Dealing Stratagems 敌战计
In wrangles between Chinese states, two Yuan brothers went to Gongsun Kang (公孙康 Gōngsūn Kāng) for help in fighting a third brother who had allied (联合 liánhé) with the powerful Cao Cao (who knew about this move). Kang beheaded (斩首 zhánshǒu) them and sent their heads to Cao Cao who made him a duke as a reward. Both had used conflict within the Yuan family to further their ends.
This stratagem is best when there are multiple warring parties, each trying to beat all the others. It can be helped by winding up individual sides, reminding individuals of grievances and so on.