A carrier can end the counterfactual war without nuclear escalation

Why not send a nuclear-armed Vanguard-class submarine back in time to stop the Second World War?

Because deterrence would require demonstrating the weapons, while a modern carrier could defeat the same forces conventionally without crossing the nuclear threshold.

The answer resists treating nuclear weapons as the automatic solution to a historical counterfactual. A submarine's arsenal cannot deter opponents who do not understand it unless at least one weapon is demonstrated or used. Modern carrier aviation already supplies overwhelming reconnaissance, reach, and conventional strike power against Second World War forces. Choosing the carrier therefore achieves the military objective without introducing nuclear devastation that the scenario does not require.