That will still need a civil war.

A regular war will do. Just point the finger at the neighbor and tell your subjects that he is responsible for gays/crops failing/drought/plague/low fps in crysis/failing birth rates/no jobs/fuel cost/you name it. See Russian invasions in all neighboring countries, the middle east, soon Taiwan etc.

Basically, they just need to mash the tribalism button until enough people are dead to suit them.