The root trouble is stated to be that two groups landed on the planet without knowing the others were there, when they met thirty years later they got a disagreeable shock.
I cannot see there was any basic difference between them, they were very similar, especially in that neither lot wanted anything to do with people they had not picked themselves.
So they divided the planet along a Great Circle which left two of the main land-masses in one hemisphere and two in another.
They agree each to keep to its own section and leave the other alone.
Twenty years later, trading like mad; each has certain minerals the other lacks; each has certain agricultural products the other finds it difficult to grow.
You think this leads to Co-operation Friendship and ultimate Federation?
I will not go into the incidents that make each side feel it is being gypped, it is enough that from time to time each has a scarcity or hold-up on deliveries that upsets the other's economy; and they start experimenting to become self-sufficient: and the exporter's economy is upset in turn. And each thinks the other did it on purpose.
This sort of situation reacts internally leading to Politics.
There are troubles about a medium-sized island on the dividing line, and the profits from interhemispherical transport, and the laws of interhemispherical trade.
It takes maybe two hundred years, but finally each has expanded the Police into an army with a whole spectrum of weapons not to be used on any account except for Defense.