G.O.D: Mezame yo to Yobu Koe ga Kikoe

In 1999, a boy named Gen explores the mountains near his small Japanese city and stumbles into an underground cave. Inside, a strange bat-like creature guards an enormous diamond. When Gen reaches toward it, the floor collapses—and in the chaos, he glimpses a group of aliens descending on Earth. He wakes up in 2009. A man in military uniform is standing over him. Ten years have disappeared, leaving no memory behind. The world Gen finds is unrecognizable: alien forces have occupied Earth, whole cities reduced to rubble, and humanity's last hope has retreated underground. Gen surfaces into a resistance movement fighting a war it did not start, and somewhere in the decade he lost is the secret of what the invasion actually is.