Aliens is a perfect example of what happens to good soldiers when you send them in undermanned, undersupported, and uninformed against a numerically and physically superior enemy who knows the terrain.

/1

A half-strength platoon, lead by a green lieutenant who insisted on micromanaging every aspect of the mission because it worked in the simulators, got its butt eaten by 187 xenomorphs, because politics allowed a corporation to dictate the parameters of the mission.

/2

Some off the top of my head tactical and strategic mistakes:

Low Morale. Vazquez/Frost, the smartgunners, and Ferro/Spunkmeyer, the dropship crew, all openly defied orders. The first pair set off a chain reaction that would destroy the colony and the atmo processor…

/3

(somewhat defensible in that they were facing superior numbers of xenomorphs and their unauthorized fire was the only thing that allowed anyone to make it out).

The dropship crew, however, decided NOT to remain on station as ordered and landed to, presumably, take a leak.

/4

This dereliction of duty on the part of the dropship crew caused their deaths, and ultimately the destruction of a significant tactical asset, stranding the survivors on the surface.

/5

Whether from a new lieutenant, a previous bad CO, or resentment at serving as mercenaries for W-Y, the half-platoon was not operating at its' peak. The troops were insolent, defied orders, and ignored briefings on what threats they might encounter.

Complacency killed them.

/6