Weapons charged and spirits high, the mech military of Montreya eagerly opened fire on the mantis invaders the moment they came into firing range. The firing range of a mech was much more limited than that of a spaceship, and so the mechs had to wait for the enemy to come much closer than the range which the fighters had engaged the mantis at.
The space station from which they had been deployed was far back, left behind so that the attack of the mantis wouldn't deal damage that could have easily been avoided if they had fought at range. As it was, the space station was several dozen kilometers away from the designated deployment zone of the mech force.
