[sliken]

Dunno, say a species starts to inhabit say 10% of a galaxy. They aren't particularly susceptible to losing a single planet and are smart (and advanced enough) to mitigate problems with the pesky upstarts.

Without FTL communications it's likely to be more of an organization than a single government, think NATO for space. Agreements for unified defense for a small fraction of the GNP or similar.

They might for instance require all ships have transponders (like ships and planes do today). Have a self replicating robot sensor that maintains the ideal density of sensors for say 1000 light years around their space (much like the current submarine sensor networks and sats monitors earth and it's oceans). Said sensors would track all ships within range, and any energy expenditures capable of accelerating mass up near the speed of light.

They would of course track their competition, but it would be far from easy for a small upstart to seriously annoy such a civilization. Even for two civilizations that each have 10% of the galaxy it wouldn't make particularly sense for them to go to war, much like today's MAD uses nukes to prevent (so far) WW3.

Not to mention hitting a planet isn't that easy unless it's on a pretty short timescale, any civilization that can launch ships at a decent fraction of light speed could likely change their planets orbital speed by 0.01% if they thought there was likely a extinction level event was heading their way in the next few decades.

[thisguy47]

Given this, it's also possible that it would take such minimal effort for such an advanced civilization to destroy upstarts, that they would automate it and be done with it.

[sliken]

They could, but it's not a good fit for the dark forest model. Sure occasionally a colony on the edge of your civilization is lost, but that happens. There's much more to be gained by not going to war and the upstarts aren't these lethal unstoppable machines.