"Communications" means more than sending a message; it includes by extension the routes over which messages, supplies, and reinforcements can travel. To have open communications means that the route is free from enemy interception. The line of communication consisted of a major supply base outside of the theater of operations, equipped with warehouses and other facilities to serve a constant stream of horse-drawn wagons or, preferably, river-bound barges. Supplies moved to forward depots, from which militarized transport battalions delivered them to the troops. During battle, the baggage wagons were kept well to the rear, near the field hospitals and vehicle parks.
Unlike the armies of the 20th century, Napoleonic armies operated without the security afforded by a continuous front. In World Wars I and II, the numerous armies each had their own line of communication. In the Napoleonic Wars, except for 1813 and 1814, there was but one army on each side operating at a given time, upon a single line. Maxim XII: "An army ought to have only one line of Communication. This should be preserved with care, and never abandoned but in the last extremity;" and in Maxim XX, Napoleon discusses changing the line of Communication. "The line of communication should not be abandoned; but it is one of the most skillful maneuvers in war, to know how to change it, when circumstances authorize or render this necessary. An army which skillfully changes its line of communication deceives the enemy, who becomes ignorant where to look for its rear, or upon what weak points it is assailable."
The advent of the railroad and industrial production changed the nature of supply in war. In World War II, there were several instances where armies lost their line of communications. On 19 November 1942, for example, the Red Army launched a two-pronged attack upon Romanian and Hungarian troops on the flanks of the 6th Army, cutting-off and surrounding the Stalingrad pocket. Hitler banned all attempts to break out; but supplying the army by air and attacks from the outside proved fruitless. After less than 12 weeks, Axis forces in Stalingrad had exhausted their ammunition and food. Napoleon's Army carried enough supply for only 10-14 days. It is often stated that unlike their predecessors and enemies, Napoleon's troops were able to subsist by foraging. This was true only as long as the army kept on moving to unspoiled territory. A brigade would exhaust the resources of its neighborhood within 3 days or less. The loss of the LOC was a morale disaster. Once the troops realized that their retreat route home had been lost, their will to fight suffered. As they continued to operate without an LOC, the lack of food, forage and firewood further abated their health and ability to resist. As the wars dragged on, generals discovered that they could continue to operate without a line of communications—as long as the countryside through which they marched was not exhausted, and a knock-out blow could still be delivered.
In 1805, at Ulm, General Mack surrendered when his communications were cut; but in 1814, when Napoleon cut the line of communications of the Silesian Army (during the Laon operation), and the Bohemian Army (at the very end of the campaign), neither army fell back, to Napoleon’s surprise.