Blue Flow

You can't learn the value of piece unless you experience war. World war breaks out after hundreds of peaceful centuries. The starting point is invasion of Allied Planetary Forces to the planet Nectal controlled by the Earth government. The defenders put up a fierce fight using giant robots as main weapons.

Main character gets trapped with his two friends during this invasion and is saved by the giant robot unit with 14 female soldiers as operators. Boys take this chance to join the defense forces. But outside the battle machines pilots are just frail girls with youth complexes and problems.

[From vndbreview]