[August]

While incompetence for the British defeat might have been the reason it was the times that exacerbated it into a massacre. Back then garrisons were cut off from each other to a degree that is hardly imaginable today. Relief for a besieged outpost was months away, there was no ELINT to know about anything about what was outside the wire beyond the range of the mark 1 eyeball, the archaic medical technology of the time could not keep the troops healthy and their weapons were not nearly as effective to mention a few differences. Had Ephinstones Army marched in 2012 instead of 1842 it would have had a wildly different outcome.

Not to say that it couldn't be done. The retreat of the 10,000 Greeks out of Persia in 401bc is a great example of what can be done with competent leaders and disciplined troops.