You wrote: >
True, 20+ years ago. Today: Spy satellites that can hide and re-position on-command. "Spy planes" are mostly Tom Clancy'esque fiction these days, at least from the perspective of over-head visual light photography. (Radio signals monitoring is a different segment.)
Then you wrote: >
False. There is no single, absolute defintion. In my experience, the Internet is "full of" posts like this one. Words and deeds are two very different things in the Real-Politik World View. Ultimately, no words matter until the "first shot is fired". See China's recent behaviour around Taiwan. Still, not a single shot fired, but a lot of potty/bully talk. (BTW -- I highly recommend China's PR mouthpiece "Global Times" as a parallel equivalent of "The Onion", vis-a-via US-China relations. Their English is but one step above Kim Jong-Un/NK media.)
Finally, you wrote: >
In the current context, the US military will probably trap the spy plane mid-air (surround with fighter jets) and force them to land in US (or US-friendly) territory. The pilot(s) would be captured as enemy combatants, similar to other spy planes downed over enemy territory over the last 75+ years. FYI: China did this in 2001. See: https://en.wikipedia.org/wiki/Hainan_Island_incident
As a thought experiment (and I do think this will happen one day): What if China parks an aircraft carrier "one meter" outside US territorial waters (East or West coast), then runs constant military exercises, 24x7, for months and months? Think more brazen: What if Chinese military jets were speed-running over Midtown Manhattan during day light hours "daring" the US military to escalate? It can, and will, happen in our lifetime. Maybe London/Tokyo/Sydney too.