I think I could narrow it down a bit. Even a single day.
6 August 1945. From that day on, the ability to destroy a city with a single bomb(er) existed. For some, anyway.
Of course since then, the strategies and tactics have been evolving to either get that bomber before he gets you, or get that bomber through, depending on perspective.
However, that which only the year before took 1000 bombers to do, could then be done with one.
And today of course, no manned aircraft at all. Perhaps the first viable ICBM was the most significant event.