I'm trying to peacefully expand my empire in Stellaris and I'm thinking of possible ways to do so.

I noticed that you can "demand vassalization" of other empires via the usual diplomatic options. My idea now is to have them join my cause first and then to peacefully integrate them into my empire later.

What are the actual modifiers for this decision? Is there a way of playing that will make other empires actually "want" to be my vassal? What would be helpful techs or traits?


1 Answer 1


You can check the modifiers by hovering over the red x (or the green checkmark) next to the demand vassalization text. It's a detailed breakdown of which factors speak for and against accepting the demand. If the total number is positive, you'll see a green checkmark and they'll accept the demand, otherwise they won't.

Here are the most important factors:

  1. Ethics. Similar ethics provide boosts to this, very important, some ethics like fanatical xenophobe will prevent a peaceful vassalization entirely. You cannot influence this.

  2. Empire strength, being stronger, i.e. having the bigger fleet and bigger empire provides significant bonuses

  3. Opinion, the biggest factor is opinion there are several ways to increase it: mutual rivals, embassy, treaties (border access, non-aggression, guaranteeing, research agreement), making the empire like you is the most important aspect of peaceful vassalization. Keep in mind, you CANNOT engage into an alliance with them, as an alliance locks you in as equal partners and disables vassalization.

It is generally pretty hard to get a purely peaceful vassalization, especially if your ethics diverge somewhat.

There is a form of military/peaceful vassalization though, that goes beyond forcing an enemy to submit. If in a war, you choose to liberate enemy planets instead of conquering them, this will create a new empire with your government ethics, on top of that, you get a massive +100 opinion for being their liberator. This usually makes peaceful vassalization of the liberated empire very easy.

  • You cannot influence this. Wait, so this game understands the concept of a society having cultural ethics, but not of influencing them to change over a long term period? May 18, 2016 at 18:18
  • Influencing another empires ethics is currently not possible besides liberating. There'll probably be an espionage dlc with options in this regard.
    – Dulkan
    May 18, 2016 at 22:27
  • Strange. In a game just last night, a friendly empire asked me to become my vassal without prompting. (We had identical governments and he'd just had half his empire taken from him by another empire I'd been fighting against.) May 18, 2016 at 23:19
  • They might do that, if they will terribly threatened by somebody else. I also had an empire, which I mostly conquered become a vassal of a big rival empire immediately after ending the war.
    – Dulkan
    May 19, 2016 at 6:26
  • Do you have any other information on the details of part 3? (I'm not looking for vassalization, but I do want to know specifically what affects this, because it also affects ‘invite to alliance’: I've got “Opinion: -1”, and I have no idea what is costing me that point.) May 21, 2016 at 1:19

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