The Western Roman Empire didn't collapse in a war, it just quietly ran out of emperors

In 476 AD, the Germanic general Odoacer deposed the teenage emperor Romulus Augustulus and declined to name a successor, sending the imperial regalia to Constantinople instead. No battle marked the moment; a centuries-long empire simply stopped having a Western ruler, the conventional endpoint historians use for the fall of Rome, even though Roman institutions and culture persisted for generations after.

— Wikipedia contributors, Fall of the Western Roman Empire — 476 AD