Suisei no gargantua anime Start with MC in a robot fighting a alien race (human race traveling to distant planet) than fail to escape with rest of the army when taking the wormhole Fall back to earth (humans live on boat bc sea lvl)
Reversed trope of : civil become pilot of robot -> expert soldier of robot become civilian Than near the end. MC discover a recording of the past, before space travel era (humanity traveling in space to distant habitable planet) And this recording is the [video] Showing that aliens, arent aliens. But ex-humans.