How this nation managed to purchased F-15 and then Su-47;
Ace Combat takes place in a separate world than ours and so all sorts of experimental or prototype aircraft see active combat and service (F-15S/MTD, Su-47, YF-23, F-16XL, etc). It doesn't make a whole lot of sense but it's a video game so like Thiel said, best not to think too hard about it.
_________________Current and Future Projects:
Ace Combat vehicles
Southern Victory aircraft
Check out my DeviantArt page
, I post cool stuff there.