Fugon, Debas, Hadyar, and the rest of the human tribes' members stood up slowly
That's right, the invaders came from another world, even if no additional reinforcements come to them since the beginning of the war, who can guarantee that they haven't already conquered a few other worlds and have troops there? In fact, isn't it illogical at all to assume that they brought their whole army and left their home world defenseless?
Even if those troops could not come easily, they would certainly come if the army in the Greenland World was at risk of being exterminated, or if they felt that their Emperor was in danger!