Trench warfare is a type of land warfare using occupied fighting lines consisting largely of military trenches, in which troops are well-protected from the enemy's small arms fire and are substantially sheltered from artillery. The most famous use of trench warfare is the Western Front in World War I.
Trench warfare occurred when a revolution in firepower was not matched by similar advances in mobility, resulting in a grueling form of warfare in which the defender held the advantage. On the Western Front in 1914–1918, both sides constructed elaborate trench and dugout systems opposing each other along a front, protected from assault by barbed wire, mines, and other obstacles. The area between opposing trench lines (known as no man's land) was fully exposed to artillery fire from both sides. Attacks, even if successful, often sustained severe casualties. However, with the development of armoured warfare, emphasis on trench warfare has declined, but still occurs wherever battle-lines become static.
Although not directly mentioned, however, some details in the manga and LN have shown that JSDF has used the trenches a lot and often. The trenches have helped camouflage the fortunes and their troops from enemy observation, for example in the second Alnus battle. In addition, when faced with direct attacks, for example, in the later stages of the Imperial Civil War. They proved effective in helping the JSDF's soldiers to hide and counterattack from a safe location in front of the superior numbers of the Imperial Army. Interestingly, it seems that at least one time Imperial Army has used trenches in battle, but their purpose is not to defend but to camouflage in an unsuccessful attempt to ambush and destroy JSDF's reconnaissance team.