This battle is amazing. And the climax is awesome. WHY DON’T THEY MAKE A MOVIE OUT OF THIS. Ok ok. WHEW. Good stuff.
The new X-Men defeat the old and they discover that the old X-Men were actually robots, a new type of Sentinel developed by anti-mutant fanatic Steven Lang. Once they learn they aren’t fighting their friends, the X-Men tear the Sentinels apart and murder Lang. All that’s left now is to escape the space station and fly home. Yay happy ending!!! (X-Men #100 - Aug 1976)