Goguryeo–Tang War vs Silla–Tang War

The Tang army had a flexible doctrine capable of shifting between field battle and siege; Goguryeo became trapped in rigid fortress defense doctrine and could not adapt to the changing strategic situation.

Silla adopted a dynamic attrition doctrine rather than static defense, effectively combining fort defenses with guerrilla offensives. Tang adhered to traditional Chinese expedition doctrine (heavy infantry and siege techniques) and failed to adapt sufficiently to changing conditions.

Attrition War

Attrition War

The Tang High Command concentrated its main effort on the capital Pyongyang while fixing enemy forces at major fortresses, denying Goguryeo maneuverability. Goguryeo dispersed forces across all fortresses, failing to create a center of gravity.

Tang's true center of gravity was the Tibetan front; insufficient force allocation to the east created strategic ambiguity. Silla directed its operational center of gravity at Tang garrisons in former Baekje territory, successfully collapsing Tang resistance there.

Tang executed a strategic deception by first conquering Baekje, placing Goguryeo between two fires. Operationally, they used feigned retreats and night raids.

Silla used diplomatic deception, declaring loyalty to Tang before launching surprise attacks. It also employed Goguryeo and Baekje insurgents to distract Tang, preventing Tang from concentrating its main forces on the front.

Tang's incendiary projectiles and heavy cavalry charges created shock effect in field battles. Goguryeo's defensive arrow barrages and sorties remained tactical, failing to generate strategic shock.

Silla avoided large pitched battles to absorb Tang's superior artillery and cavalry shock. Instead, fort defenses and night raids neutralized Tang's firepower. At sea, the Silla navy broke Tang's amphibious shock capability through coastal defense and estuary ambushes.

Goguryeo used mountainous terrain and harsh winters to exhaust Tang armies defensively; as seen at Ansi, even artificial terrain tactics were countered by natural elements. However, storms and supply issues at sea affected both sides.

Korea's mountainous terrain and harsh winters offered natural advantages to Silla's defensive tactics. Tang's campaign seasons were limited by frozen rivers and impassable mountains; Silla turned this difficult terrain into a force multiplier with interior-line maneuvers.

Tang, through Silla intelligence and its own diplomatic channels, learned Goguryeo's defensive weaknesses, while Goguryeo failed to anticipate Tang's main axis of advance and the prioritization of Baekje.

Silla gathered critical intelligence via students and merchants in the Tang court; for example, prior warning of the 670 invasion plans enabled organized defense. Tang failed to adequately analyze Silla's internal dynamics, especially coalition cohesion.

The Tang army advanced rapidly using interior lines and executed flanking maneuvers with agile cavalry. Goguryeo relied on static fortress defense and failed to seize strategic counterattack opportunities.

Silla leveraged interior lines to rapidly shift forces from one front to another. Tang's slow, overseas movements on exterior lines were ineffective against Silla's maneuverability. The swift capture of forts in the Baekje region exemplified this flexibility.

The Tang Emperor's personal presence and early victories boosted morale, but failure at Ansi caused a morale collapse. In Goguryeo, Yeon Gaesomun's charisma sustained morale until his death triggered a complete breakdown.

On Silla's side, a sense of national resistance against a foreign power kept morale high. Tang soldiers suffered from an unclear strategy and war-weariness away from home. Silla's common cause as coalition leader cemented psychological superiority.

The Tang Empire isolated Goguryeo by allying with Silla and conquering Baekje; it also fostered internal collapse after Yeon Gaesomun's death, effectively winning without direct battle in the final phase.

Silla gained strategic advantage without fighting by deepening Tang's internal political pressures (Tibetan threat, court intrigues) through diplomatic maneuvers. The tributary mission in 675 successfully diverted Tang's attention westwards.