This one thing was always troubling me. How and why could the Belorussian and Ukranian SSRs gain so much of theoretical autonomy so as to have their own autonomous Foreign Policies while other states couldn't?
It was a diplomatic fiction. The UN was set up to represent the international power balance at the end of WWII, not just the ideals of democracy, equality, and freedom.
The capitalist winners of WWII had three permanent seats in the security council, four is you count the RoC as capitalist, while the communist winners had just one. Similar numbers in the general assembly. So the Soviets got two extra delegations with extra votes in the general assembly, but no extra veto powers -- one veto is enough to stop any action.