Godzilla: City on the Edge of Battle

Following their encounter with Godzilla Earth, an evolved version of the monster that wiped out Earth civilization over 10,000 years ago, Haruo Sakaki and his surviving platoon encounter the "Houtua", a tribe of the remaining descendants of the humans on Earth. Upon the discovery of nanometal that evolved from the failed Mechagodzilla project, the platoon stays on Earth against the Central Committee's orders to retreat and enters Mechagodzilla City to plan their next strategy against Godzilla Earth. However, the Houtua warn Haruo of impending danger within the nanometal city.

Synopsis from Anime News Network