I finished watching Iriya no Sora, UFO no Natsu yesterday and left me with unanswered questions

First question is, who is Iriya really fighting with?
This anime kinda reminded me of Saikano where I had no idea who Chiise was fighting with in the first place
Both animes were fighting against an unknown enemy (at least to the audience) for the survival of Earth

If I translated it correctly, I believe the war was between humans who secretly made UFOs (correct me if I'm wrong)
But Iriya is the only one left to pilot the Black Manta (or whatever it was called).... so who was she fighting with?
Or was she fighting with aliens?......

Also in the end, when Asaba convinced her to fight again, what happened to her?
Did she die?
I didn't get what the letter that Asaba recieved in the end was referring to

Hope somebody can explain to me