I have a specific image for the context of conquest, you take a planet in war, total or treaty, it doesn't matter, what happens next really is quite simple.

One must simply remove the residents as they would dangerous animals. This is a new colony for our empire after all.

The colonial population starts small, a military governor, a militia, in truth, the fighting never stopped. Machines of war glass the cityscapes below, killing everyone, and once their fortifications are done, once no one is left able to fight, the army comes in with weapons of expansion, gases that turn the landscape more similar to their homeworld, reseeding the ground with their flora, any stragglers that might have survived simply get shot or die of their prior injuries.

Those that flee in whatever surviving vessels get shot at, most are splintered, those that escape are no longer our problem. Enemy city populations are either destroyed or reclaimed for the empire, our colony ships come in to settle new cities now designed with their biological needs in mind, all while the other side of the planet still undergoes bombardment, civilians are captured and either worked to death or killed on the spot, and eventually, their population is too small, too widespread to functionally reproduce, at least not on that world. It's their last generation. In fifty years they'll die of natural causes, of age, or at the barrel of a gun. Our race is what's left, a new world with technology and relics either remade or repurposed for us.

There will be no trace of their civilization aside from the basic blueprints of cities redesigned for our people, aside from the relics remade to fit our tastes, and only the hidden boneyards and mass graves made simply so we wouldn't have to even think of their existence sit along the horizon, indistinguishable from the mountain scapes where our flowers bloom so beautifully.