Ai wars & omniscience

In the options screen there is something like
view full battles
and
always watch opponents battles
not sure if thats exactly how its worded, but that should let you watch what happens in other civs battles (don't know I've never turned it on) Don't know if thats what you're looking for, or if you mean you want to play in a spectator type mode where you don't play, just watch. Don't know if you can do that, but good luck.

That's not what he wanted.

He wanted a Computer Vs. Computer full-on war. Not just to watch the battles.

As far as I am awear it is not possible.

Technically, if you research omniscience, you should be able to see it. But NOOoo!

You could build a vast network of sensor-laden ships to watch the fighting.

...not sure if that's the sort of "watching" you're looking for though.

im thinking of the same kind of thing as in
https://forums.galciv2.com/index.aspx?AID=141466#1110661

Not half-baked get a lot of sensor ships kind of thing

plus doing that is much more likely to overload your system; that's the whole reason sensor range was limited in the first place.