Our armies were penetrating the enemy's country in close proximity to a determined and vigilant foe.

Every scout, every skirmish line, every picket, every foraging party ran the risk of falling into a Rebel trap.

This was in addition to the risk of capture in action.
The bulk of the prisoners were taken from the Army of the Potomac.

For this there were two reasons: First, that there were many more men in that Army than in any other; and second, that the entanglement in the dense thickets and shrubbery of the Wilderness enabled both sides to capture great numbers of the other's men.

Grant lost in prisoners from May 5 to May 31, seven thousand four hundred and fifty; he probably captured two-thirds of that number from the Johnnies.
Wirz's headquarters were established in a large log house which had been built in the fort a little distant from the southeast corner of the prison.