Like all peoples of Antiquity, Roman troops used war cries to frighten the enemy, demonstrate strength and eagerness, and heighten individual and collective determination, but the demands of discipline and tactical cohesion required them to exercise restraint. Frequent shouting generated alarm or impetuosity among both men and horses, and impeded the communication of orders. Accordingly, battle cries were only permitted immediately prior to or upon engaging the enemy at close quarters. The strict observance of silence until this juncture also unnerved opponents and intensified the psychological impact of the battle cry. This practice is documented during the Principate (e.g., Arr. Ect. 25; Cass. Dio 62.12.1; Cowan 2007). In Late Antiquity, tactical treatises continue to recommend similar procedures (Veg. Mil. 18.104.22.168.10; Maur. Strat. 12.B.14.412.B.14.5, 16.3916.44). Maurice requires the rearguards or file-closers to jab with their spear-butts any soldier who so much as whispers during the advance into battle (12.B.17.3912.B.17.44). For cavalry formations, he considers noise so detrimental that he forbids war cries and bugle calls until after the battle has begun; then mainly the rear ranks may shout or roar to urge on their comrades and panic the enemy (2.172.18). Roman training probably included instruction in the correct use of battle cries (SHA Alex. Sev. 53.853.9; Theoph. 303.12303.17). Similar procedures are reported in historical accounts (Amm. Marc. 16.12.43; 21.13.15; Procop. Wars 4.11.36, after Thuc. 2.89.9), although it is unclear to what extent this ideal was always achieved (e.g. Amm. Marc. 31.7.11). Even if disciplined silence versus disorderly clamor came to reflect a wider rhetorical distinction between Roman and barbarian, it nevertheless seems to be an accurate portrayal of differing military psychologies. In contrast, the empire's chief opponents habitually employed terrifying war cries (e.g. Dexipp. FGrH 100 F26.6; Amm. Marc. 27.10.10; 28.5.6; 31.7.11, 12.11), notably the shrieks of the Huns (Amm. Marc. 31.2.8) and lupine howling of the Avars (Men. Prot. fr. 12.3; Gregory Tur. HF 4.29; Suda 804) and Slavs (Ps.-Caesarius 109; Maur. Strat. 11.4.5311.4.6). In these circumstances Roman self-control was a considerable feat, especially when deployed alongside less-disciplined allies (Th. Sim. 5.9.57).
In the 4th century, Roman infantry favored the barritus, a war cry of Germanic origin, apparently imitated from a martial custom prevalent among auxilia palatina from East of the Rhine. It began as low murmuring and gradually crescendoed to a loud roar (Amm. Marc. 16.12.43; 21.13.15; 26.7.17; 31.7.11; Veg. Mil. 3.18.9; Lactant. Plac. 4.394). The etymology of barritus is ambiguous. Tacitus (c.98 CE) used the term barditus to describe the manner in which Germani chanted songs (carmina) in the battle line, which amplified and reverberated within the hollows of their shields (Germ. 3.1). The late Latin form most plausibly evolved from a deformation of barditus under the influence of an existing Latin word barritus, a bellowing or trumpeting (Apul. Flor. 17; Veg. Mil. 3.24.5; Hoffmann, SBND 1.1357; Beck, 1976; Speidel, 2004: 111113). Later, battle cries increasingly took the form of Christian invocations, notably Deus nobiscum, God (is/be) with us (Maur. Strat. 2.18), and perhaps also Alleluia (Vita S. Germani 3.18). Towards the end of the period, Adiuta Deus, God, help us, was officially sanctioned (Maur. Strat. 12.B.16.4212.B.16.43, 24.1524.16; Lot 1946; Rance, Strat.). If victory was apparent, other slogans proclaimed the triumph of the emperor (Veg. Mil. 3.5.4; Procop. Wars 2.8.29, 30.3). On a related theme, Vegetius does not specify the wording of battle cries but mentions Deus nobiscum as a likely watchword (signum) used both during sentry duty and in battle to differentiate friend from foe (Mil. 22.214.171.124.5). The use of such watchwords as a means of identification in combat is attested in the 6th century whenever identical troops were fighting on both sides (Procop. Wars 4.17.22; Th. Sim. 126.96.36.199.5).