As I said, the US ran a surveillance/SIGINT mission, well in international air space. Nothing unusual about it.
The Chinese, who consider the Hainan Island area very sensitive, intercepted the US aircraft, also in international air space. Nothing unusual about that on its face either. They are as free to fly into international airspace as the US is.
Now, clearly the US felt that the Chinese aircraft hazarded the US aircraft...and so the US is protesting. That is their right. They may also send a message, just like the Chinese did...with another carrier and possible escorts. They are completely free to do so in international air space and waters.
The outcome may continue to escalate. Lets hope not. We will have to see.
But this type of intercept happened during the Cold War...not as in every month or so, but over the course of twenty years...a lot of times.
These types of flights, when one major power wants to gather information on another, is not a risk free thing. Never has been...I do not think it ever will be.
The fact is, if the US thought that the PRC was about to discover very classified information while flying close to one of our most sensitive bases...we would respond to warn them off. Perhaps not with what we might consider a dangerous provocation (ie. a barrel roll over the surveillance aircraft)...but the Chinese might think whatever warning we gave was.
They then would be free to protest, and/or send a stronger message about it...just as the US is doing. This is not outside of a normal sequence of events in geopolitical circumstances.
To date, no one has been injured. The US aircraft was not forced to land, or even...to my knowledge...forced to abort.
Also, I must add, when you look at the location, it is also clear that this had NOTHING whatsoever to do with the ADIZ that is the purpose of this thread. The ADIZ is north of Taiwan. This happened off of Hainan Island, almost 1,300 kilometers to the south west of Taiwan.