What reason would the Soviets have to 'take over China'? This is completely irrational.
As irrational as the USSR joining the Axis.
Add it up. Japan was at war with and engaged in an ongoing conquest of China. Japan is part of the Axis, which puts China on the side of the allies.
Japan dukes it out with America, there's no way it can win. It's fleets are destroyed, its an island cut off from support, and atom bombs get dropped on it.
But it's Chinese holdings are up for grabs. The Soviets, who are not at war on their western front, sweep in through the thinly populated northern territories of China, link up with orphaned Japanese armies, and voila, they've conquered China. Or at least, that's probably the theory they're going to be selling each other.
Short of outright genocide, it would be completely impossible to police a hostile population of such proportions;
Yeah, that's why the Mongols and Manchu and Japanese never bothered to invade China. You're right.
and if a union of these two countries was somehow established on an equal footing, the centre of politics would almost automatically shift from Russia to China.
Because the USSR was so strongly committed to things like one man, one vote; free elections; and not looting the crap out of captured nations. Agreed. As historical precedent, we can cite the shift of the British Empire's capital from London to Delhi, a controversial move at the time.