[Yizahi]

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.

[twojacobtwo]

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