The question of "When did diplomacy ever work?" really depends on the objective of a given diplomacy. There were diplomatic missions with the specific objective of starting a war. If hostilities broke out, that diplomacy succeeded excellently.
The mistake is a modern notion that diplomacy is the antithesis of war, and that the sole objective of diplomacy is to prevent war. This is not true at all. Diplomacy is nothing more than the communication between states. Sometimes that communication becomes physical, which is why war is referred to as an extension of diplomacy by other means. As long as communication between the states is ongoing (and carpet bombing another state's cities is EXCELLENTLY clear communication, wouldn't you say?) diplomacy is "working".
We are accustomed to the messages that diplomacy seeks to impart being nice and pleasant and reasonable. This is because generally countries don't want wars, it is bad for them. But sometimes the message countries want to impart with their diplomacy is "STFU, I will pwn u".
-Andrew