So let me first explain a little bit as to what a supply line would have looked like. It would have been a massive wagon train, stretching out across the road filled with laboring beasts, sweating workers, and stench of feces and a great unwashed mass. Jostling underneath wagon covers would be every conceivable item needed for war, from ammunition and food to water, boots, hats, coats, blankets, haversacks, bayonets, swords, guns, picks, shovels, wooden planks, replacement parts for other wagons and literally every implement soldiers needed to stay effective in the field. Even soldiers themselves, destined for the frontline armies, used these routes to move from supply areas in the rear to the armies up front. But, like the armies of the time, these wagon trains were pinpricks in a vast arena. And they werent the only way armies transported goods.

So whats a supply line? Its best to think of them as major highways for moving men and materiel from depots in the rear to the armies up front. These supplies were usually transported in one of three different vehicles: trains, boats, or by foot power (either a beast pulling a cart, or a man carrying goods, and himself!). The Civil War was the first major conflict fought which was aided by the use of steampowered locamotives. In the realm of tactics and operations, these trains allowed armies to quickly shuttle forces across the vast American theater. They also allowed armies to reinforce and resupply quickly as supplies moved away from rear area depots to the railheads, which were the closest railway point to the front line, adjusting for the conditions of other transport mechanisms.

Railways also are the easiest supply line to understand. You have a thin ribbon of steel across which the lifeblood of armies transit. Where the railways were well developed, like in the East, transit was quick, efficient, and voluminous. But in less developed areas, or where the rail line decayed through overuse, volume quickly outstripped capacity, and prevented the free transit of supplies. Imagine what a damaged bridge, a wrecked train, or a separate rail might do to a busy rail schedule. Now imagine that same scenario in an area which was poorly developed, where that accident could block a critical passageway, and could entail rerouting traffic many hundreds of miles to reach the same destination.

Road and river traffic was more secure and less prone to accident, sabotage, and blockage. But thats not to say that there werent chokepoints and vulnerabilities across all forms of supply traffic. Each individual supply column was hard to find without the effective use of spies, scouts, and intelligence organs. Cutting a supply line rarely entailed the actual physical destruction of supply convoys and depots (though that did happen, especially in the case of raider warfare). Rather, armies would attempt to lay themselves across the enemy's supply lines, garrisoning key junctions and preventing the free flow of materiel. This would have one of three effects. If the enemy army kept on attacking, it would do so using only what goods it had with it. In that situation, most armies would simply shrivel up and die. Instead, the enemy army could pull back, attempt to outmaneuver their attacker, and reunite with supply lines. This is a dangerous move, however, because the enemy already controls some of the best routes back to base. Finally, the enemy could turn and attack, attempt to dislodge the blockage and reunite with supplies. No matter what strategy a general picked, he would have to do so with only what was available at that moment. Further, the general doing the cutting would be the one calling the shots. He controlled vital points on the map and narrowed down the possible responses greatly.

Outside of the major campaigns and set piece engagements, Raiders also plagued Union supply lines. As the Union army moved into the Confederacy, they moved away from their bases. In many cases, this led to a long and perilous supply chain which had multiple vulnerabilities. Confederate raiders, usually cavalry, would circle around the Union armies (again, pinpricks in a much larger theater) and move up the supply line. There they did as much damage as they could. Rivers were the hardest to plug up, but sunken ships and steaks could block river travel. Felled trees, burnt bridges, and roving gangs all plagued back area roads. Railways were the easiest to destroy, however, because men could easily pull apart miles worth of track, and even the removal of one rail could stop an entire train until it was repaired. All the raider's had to do was ensure that enough damage was done that could not be easily or quickly repaired. To counter this threat, the Union expended vast amounts of manpower to secure their supply lines. Between half an armies paper strength or more could be devoted just to securing the supply lines or rear areas. These forces were used in a variety of ways. Some marched with convoys to repel raiders, while others patrolled road and rail hoping to catch saboteurs in the act. In other places, they occupied garrisons to provide refuge for traveling wagons and convoys. But in every place, raider still had success. Further, rear area work could be dangerous. Confederate raiders were especially nasty, and regularly picked off isolated Union patrols.

Logistics and supply are one of the unsung elements of modern war. When we think of great military victories we rarely remember the supply clerks and logisticians who enabled that victory. Even armies which lived off the land still consumed some supplies. Men and ammunition were always in high demand. So the armies and commanders of the Civil War constantly considered where there army would get its next meal, and where it would get its next bullet. The supply side of the war was just as revolutionary, and just as important, as the major engagements.

Sources:

Russell Weigley, A Great Civil War

Gordon Rhea, The Battles for Spotsylvania Courthouse and the Road to Yellow Tavern

Martin van Creveld, Supplying War