You're trying to draw a distinction that doesn't exist.
The Allies said to Germany they wanted unconditional surrender. The Germans agreed. Various terms were agreed to including the cessation of military operations (on both sides). This is, by definition, a negotiated agreement ie diplomacy.
Putting the highly questionable ethics of the two atomic bombings of Japan aside for a second, these bombings like any military operation are ultimately a negotiation tactic.
The Allies said to Germany they wanted unconditional surrender. The Germans agreed. Various terms were agreed to including the cessation of military operations (on both sides). This is, by definition, a negotiated agreement ie diplomacy.
Putting the highly questionable ethics of the two atomic bombings of Japan aside for a second, these bombings like any military operation are ultimately a negotiation tactic.