Dreghis are what they are, kind of a Golden Horde of the galaxy. They always go to war with everyone and I have had a game where they threatened me before I even met them, but this time was different.

They didn't attack me at all. The relations were "furious" all the time, they attack everyone else, but not me (Terrans). I could attack them myself (I checked and it worked normally), but instead, I asked them to pay me tribute (first time when I did it they had 2x my military power), then I started building cultural starbases next to their planets. More minuses in diplomacy with them, but no war.

They were at war with everyone else in the galaxy though.

I thought it was because of "interstellar republic" government I had at one point with "can't declare war" trait, but after changing to "singularity" no change. 

I went all the way to research victory without a war this way. What can be the reason?