This question already has an answer here:
In Starwars during the battle of Hoth, the empire made what i believe to be a hilarious strategic miscalculation and sent vehicles with only forward facing firepower (unimpressive firepower at that), are easy targets, and are slow moving, up against the stronger, more agile squadrons of rebel fighters which consequently result in massive imperial AT-AT casualties.
The rebels had previously illustrated a proficiency at fighter based combat and rely upon them heavily. Fighters appear to be the perfect counter to an AT-AT assault.
So why would any general in his sane mind deploy them? granted, i recognise that their firepower was sufficient to breach the rebel defences, would likely impose a significant psychological effect on the enemy, would act as overwatch due to heigh and acted as troop carriers. However they were completely vulnerable to the obvious rebel fighter counter and appear to be totally impractical anyway: They are slow moving, easy targets and possess obvious weak spots around the cockpit and leg joints.
What ever happened to tanks, which would be the far more logical alternative, especially when supported with ground to air/air-air defences, and why would the empire use AT-ATs?