with which to fight the war and thus could afford to loose men. Grant knew this and knew it well, which is one of the key reasons he stopped the prisoner exchange. This meant that not only was the Confederacy losing men to death. but it also lost men to the prisons in two ways. First, of course, was that men became POWs and were not exchanged so as to be likely to return to the battle line. The second was as prison guards. With more and more prisoners coming in on both sides, the prisons had to hold more men and thus have more guards to increase to chances of preventing escapes. Again, with the Union's advantage in manpower it could afford to put some men to the task of prison guards. The Confederacy, on the other hand, could not afford this.
Time. Time was the key reason for the North's victory and was achieved through a combination of the first and second reasons. As the war progressed the Union's blockade, largely a paper tiger at the beginning due to the fact that the Navy didn't have enough ships to properly enforce it, became a real blockade that slowly strangled the Confederacy to death. And as the war progressed the South, even before the end of the prisoner exchange, was losing men it could not afford to lose to attrition. Supplies and transportation of supplies. As stated in the first reason, the North was less dependant on Europe than the South was for supplies. There should be little doubt that the South was able to manufacture supplies needed to fight the war, but never in amounts it needed. Industry in the North was a bit larger than it was in the South and was thus able to out produce that of the South. But being able to out produce the South was all well and good unless the South could get the supplies to its troops in the field faster. It then became important to capture major railroad junctions and thus cut off the South's ability to move supplies in a timely
fashion. At the same time the capture of major railroad junctions helped to speed up the supplying of Union troops in the field.