The figurative notion that circumstances, relationships, or global trends are in a continuous state of flux and alternation.

商业伙伴之间的关系也是分久必合，合久必分，没有永远的朋友或敌人。

The literal historical cycle where entities long divided eventually unify, and those long unified eventually divide, as popularized by Romance of the Three Kingdoms.

中国历史的发展规律往往被概括为“分久必合，合久必分”。