France wouldn't be about to publicise their involvement, and would anyone else really be in a condition to determine where the strike came from, especially if fired from a sub?
France's move into the middle east and Africa wouldn't have happened on the spur of the moment either. They've clearly had time to observe what's going on around the world and make plans. Very likely any overt move on their part would have been preceded by months of build up and covert action.
That said, I doubt it was them. More likely in my opinion the Soviets used low yield tac nukes on Romanian troop concentrations rather than infrastructure. We know that the Romanians took out at least one Soviet Division (the 280th) by ambush (albeit one wracked by desertions, etc). What better way to neutralise them than by making them glow in the dark? A handful of nukes is also a lot quicker than moving other units into the area for conventional operations and would be able to be carried out before the Romanians could disburse.
Hmm, evil thought - what if the Soviets used the 280th as bait? It was a mobilisation only unit and not exactly effective anyway.... Timeline probably doesn't work too well, but...
