Most science fiction prefers to ignore the speed of light when thinking about space colonization. Star Trek has warp speed, The Expanse has wormholes, Three Body Problem has instantaneous interstellar communication. If you could travel faster than light, a civilization that stretches across multiple star systems might feel just like a civilization that covered multiple continents on Earth, in the days before jet travel.

In reality, we’re pretty sure that you can’t go faster than the speed of light. What does this mean for the politics of an interstellar civilization?

Imagine humanity spread across just two planets, that are 20 light years apart from each other. You can communicate, but it’ll take at least 20 years for a message to cross. You can travel yourself, but it’ll take at least 20 years for a ship to cross.

Can a single country span the two planets? It doesn’t really make sense - a ruler or a legislative body in one place would take 40 years to handle any issue that arose. 20 years to learn about the problem, 20 years to inform the locals of the solution.

How would war between the planets work? Imagine you lived on one planet, and you heard that the space Nazis had taken over the government of the other planet. They hate your planet, they haven’t declared war, but they have enough nuclear weapons to destroy your planet. Any moment now, a swarm of nukes could drop down from space. Maybe they launched 19 years ago and they’re already on their way.

Mutually assured destruction works when one side can’t wipe out the other, and both sides share a planet that they don’t want to destroy. If one side has superior technology, so that they could win with a first strike, the logic of mutually assured destruction doesn’t apply any more. But with distant enemies, you don’t know how much their technology has advanced in 20 years.

You also can’t understand the mentality of your enemies, if they live 20 light years away from you. You don’t even know the names of the decisionmakers. You can’t pick up the red phone to Moscow in case of emergency to talk people down from the cliff.

You can easily imagine a world government learning that bad guys were about to take over a distant planet, and deciding that a preemptive war might be necessary. Of course, if both sides are thinking this way, that just makes war even more likely.

It seems quite dangerous to settle planets in other star systems before we have figured out a way to stop having wars. Fortunately, it looks like it’s going to take us a while to spread out of the solar system. Hopefully we can figure out this “peace on Earth” thing first.