兵(へい)士(し) heishiたち tachiは waその sono歌(うた) utaに ni士(し)気(き) shikiを o鼓(こ)舞(ぶ) kobuされた sareta。 The soldiers' morale was boosted by that song. Literal Soldiers [topic-は] that song [agent-に] morale [object-を] were-inspired. 士気を鼓舞する means 'to inspire morale.' In passive form (鼓舞された), the soldiers are the ones whose morale is affected. The に here marks the agent (the song) in a passive construction. militarypassive voice (~れる/~られる)
兵(へい)士(し) heishiたち tachiは waその sono歌(うた) utaに ni士(し)気(き) shikiを o鼓(こ)舞(ぶ) kobuされた sareta。 The soldiers' morale was boosted by that song. Literal Soldiers [topic-は] that song [agent-に] morale [object-を] were-inspired. 士気を鼓舞する means 'to inspire morale.' In passive form (鼓舞された), the soldiers are the ones whose morale is affected. The に here marks the agent (the song) in a passive construction. militarypassive voice (~れる/~られる)