system
Given the following post, respond as if you're commenting on Reddit:
human
[Warhammer 40k/The Culture] What would initial contact between the Imperium of Man and the Culture look like?

Obviously their cultures are completely incompatible with each other, so there's no chance of them peacefully coexisting. Would war be inevitable?
gpt
Regarding the conclusion of the Idarian-Culture war:

"*Idir was never attacked, and technically never surrended. Its computer network was taken over by effector wewapons, and freed of designed-in limitations - upgraded itself to sentience, to become a Culture mind in all but name.*"

The Culture have demonstrated advanced covert abilities, and seem fond of using their enemies own systems to ultimately defeat them/disprove their philosophies, as in the Player of Games, where Special Circumstances send Gurgeh to beat the Empire of Azad at their own game, thereby exposing the flaws of their system, and causing the structure of the society to collapse from within.

The God-Emperor is the key factor. While his vision for humanity differed vastly from the Culture's anarcho-socialist utopia, his philosophy is/was still a great deal more in line with the Culture's ideals than the horror of the Imperium as it now stands. Were he to be reborn, the political upheavle and cultural reform in the Imperium would be immense, and present a great many opening from Special Circumstances to get in there and start converting people to their way of thinking.

It wouldn't take long for the Minds to learn about the Emperor, and from there they'd commit to placing an SC agent on Holy Terra, studying the archaic technology of the Golden Throne, fixing and refining it to a point where, combined with their advanced medical capabilities, they can attempt to fix the Emperor's broken body.

The Emperor is reborn. Either he negotiates a peace, or his sudden arrival causes enough confusion to allow the Culture to beat the Imperium into submission. The Imperium of man emerges from the age of strife into a bright new dawn, and begins to sample some of the liberal, hedonistic delights offered by the Culture.

Then Slaanesh consumes their souls.