World War II was a much more mobile war than the First World War, particularly in the West. That being said, the usage of field fortifications was standard military practice for all sides, with troops digging a variety of defensive positions for a variety of different reasons. Hardened, static fortifications were also used extensively, particularly by the German Army along their western border with France, along the Atlantic Coast in Occupied Europe, and in Italy, but these were features of preplanning or slower paced fighting.

Terrain is a dominating element of warfare. Open fields and rolling hills provide avenues for armor to maneuver, with forests, hostile cities, and mountains restricting that ability and returning the brunt of the fight to the infantry. The First World War ground into stalemate in 1914 due to an imbalance between firepower and mobility- horses and men couldn’t outrun machine guns. Over the next few years technological, tactical, and organizational advances evened the playing field- tanks came into being, the firepower of attacking infantry was augmented at the platoon and company level, communications between infantry and artillery was improved, and combined arms tactics were pioneered, to name a few- with mobility being restored to the battlefield through combined arms action in 1918.

During the Second World War, advances in technology, tactics, organization, and communications ensured that well planned and executed combined arms attacks were capable of defeating static defenses in most if not all scenarios, thereby preventing the recurrence of static warfare on a major scale. Therefore for combatant soldiers actively involved in combat (there still existed lulls between campaigns where frontline troops did little more than patrol, fortify their positions, and fight limited small-unit battles) the war was one of constant movement and grotesque routine: wake up, attack during the day, and dig in at night or when the attack petered out. The former usually took the form of foxholes or slit trenches- the depth of said position varied on unit leadership and individual enthusiasm and energy, in the sense that well-led and disciplined outfits dug in deep enough (and dug the appropriate position- much more often a foxhole than slit trench) to ensure safety should a concerted attack be launched against them, while poor leadership could lead to units succumbing to fatigue, with the result that their fighting positions were shallow, poorly sited, and potentially inappropriate for beating off a counterattack. The latter, positions dug once an attack had reached its climax and failed, were whatever depression a man could scrape out under fire, and usually improved at nightfall. In terms of distance from the enemy, it all depended- take, for example, the Lorraine campaign in the fall of 1944. Lorraine is comprised of open, rolling farmland interspersed with wooded high ground. In attacking through such terrain armor and mechanized infantry were utilized over open ground with standard infantry tackling the woods. In such a fashion, as an infantryman following a successful attack to clear the woods the position you would occupy would be overlooking open ground- chances are the nearest enemy troops of any size would be in a village below you or on the next hill or ridge. Therefore the “no man’s land” for that day could be as expansive as a mile or two. Likewise, if your attack through the woods failed to drive the enemy out of the forest, you could be digging in for the night at a distance of 10-20 yards from the nearest enemy outpost or even strongpoint. Depending on the specifics of the situation, you might be pulled back through the woods anywhere from a few dozen to a few hundred yards to improve defensibility or simply reorganize to resume the attack the next day, at which point the distance between the lines would have been opened accordingly. The same type of scenario can be found in urban fighting, with the lines potentially being as close as house to house or located on opposite sides of the street. More often, in successful urban attacks American troops would stop at predetermined phase lines set by prominent buildings or main thoroughfares, thereby opening the distance somewhat as these lines were specifically chosen partially for their defensibility against infiltrators.

The same relationships can be found in mountains and desert. The more expansive, open, and less defensible the terrain, the greater distance between the lines, potentially miles or dozens of miles, while in mountain fighting you might have two forces separated by ten yards and lobbing grenades at each other around the clock. Atoll and small island fighting in the Pacific was a little different, and featured very close lines due to the cramped nature of the battlefield and the characterization of the mission as a continuous assault.

I’d be glad to give you more specific information from the American perspective if you can give me specific periods and places that you’re curious about. I am not very familiar with the Soviet aspect, so if you have questions about the Eastern Front someone else will have to jump in.