It's simple really. What if Belgium tried to pull a 'France'/Portugal and use force to keep the Congo instead of granting it independence? I don't know if this is an accurate comparison but the Netherlands (TINYcompared to it's colony of Indonesia was able to fight against the rebels for a good while all the way from 1945-1949 IIRC..I'm sure international pressure would eventually be brought down on Belgium to let the Congo go, but would the Soviets try to find a way to support the Congolese?