What if the UNSC found a Forerunner dreadnought in 2470, similar to the ship at High Charity?
Short answerIf the UNSC could operate it, the resulting technological advantage would be overwhelming against the forces attacking human space.
NotesThe scenario gives the UNSC a Forerunner dreadnought decades before the later war. Successful operation is the decisive condition. If humanity can get the ship working, its attackers face an overwhelming opponent. Simply discovering the hull does not satisfy that condition.