By the time Plymouth fell, the armies in the East were on the eve of the Overland campaign and its intensive use of field fortifications. The preceding campaigns from Big Bethel to Plymouth were in one sense a preparation for the habitual use of fieldworks in 1864–65. Commanders on many levels relied on breastworks, earthworks, or preexisting features on the battlefield during almost every significant engagement from 1861 through 1864. There was a definite trend toward greater reliance on fortifications, but it was not steady or inevitable.
The evolution of trench warfare was centered, in part, on the problem of balancing the desire for offensive action with the need for assuming the defensive. If commanders wanted to take the tactical offensive, they often refused to dig in. Entrenching was, by definition, a sign that the commander wanted to hold his position and fight a defensive battle. Unless they were specifically designed to maintain one position while the commander attacked from another, trenches locked soldiers into a static defensive mode. Often, if the commander were undecided, he would refrain from entrenching in order to keep open all his tactical options until the last minute.
There was an ebb and flow in the growing reliance on field fortifications in the eastern campaigns. They were used from the beginning but did not necessarily play a decisive role in the outcome of either campaigns or battles. They were often used right after an engagement rather than before or during it. One can see many examples of this, especially after First Manassas and Fredericksburg, where the shock of combat led to a greater desire for cover in case battle was offered again quickly.
The Peninsula campaign saw a great deal of digging by both sides. McClellan recognized that the use of earthworks could help him achieve strategic results while lessening casualties. The Confederates relied on strong earthworks to delay him during his advance up the Peninsula. Lee also relied on extensive fortifications as a preparation for his offensive against McClellan in late June.
Ironically, the success of the Seven Days made the average Confederate soldier, and Lee himself, less appreciative of earthworks. Their offensive