I doubt Grant would have need additional troops had the Confederates tried to hold Donnelson. He had them surrounded with no source of resupply. He, on the other hand, could be well supplied from the naval forces on the river. Seems like all he need do was wait them out.

As for a rescue operation, hard to say. From what I've read I say unlikely. IIRC, Johnson was reluctant to reinforce Donnelson and only did so under political pressure from the Gov. of Tennessee.

There were something like 15,000 troops over at Columbus but getting to Donnelson might have been very difficult. Roads were virtually non-existent and they would have to cross the Tenn river also. Guess they could have come down the railroad and then go back up through Paris to Clarksville.

Good question.

Tom