Coming back together again after being separated or in conflict is called reunification. This noun is usually used to describe relations between countries, states, or territories.
It's not uncommon that two countries will be allied and then find themselves clashing or even going to war against each other. If they eventually join forces again, it's an example of reunification. In recent history, the merging of East and West Germany back into one single country, in 1990, can be called reunification. Reunification, from the verb unify, comes from the Late Latin unificare, "make one."