Black Star Rising (1986), by Frederik Pohl?
This review matches nicely:
Once the western powers have annihilated themselves in a nuclear war, India and China divide up the world between them; what's left of the U.S. becomes a Chinese agricultural colony. Then rice-paddy coolie and aspiring scholar Pettyman Castor discovers a severed head in the mud--and the police investigator, lustful Hah Chinese inspector Tsoong Delilah, conceives a passion for him.
During the official enquiry, Castor is given the opportunity to use an advanced computer and information network; astonishingly, he discovers that an alien spaceship is approaching, and demanding to talk to the President of the U.S.! Prodded by Delilah, the Chinese hurriedly name Castor President and send him into space to meet the aliens--where a ""spaceway"" transporter whips Castor instantly off to World, a planet inhabited by mammaloid-insectoid ""erks,"" plus a few thousand jingoistic Americans, descendants of an expedition dispatched just before war broke out.
At this point, though, the narrative loses some momentum and focus among several different points of view. The Americans and their erk allies are a-lather to liberate the U.S. from its Chinese oppressors; Castor goes along with the idea, until he discovers that the erks have made a profession of war--and that none of the many other races ""liberated"" by the erks has survived.
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[story-identification] aliens Chinese which bought up Looking for the name of this book. China rules the world when aliens make contact, they want to speak to the US president..