I'd like to reiterate some points I made earlier and reopen the discussion on how the D&D rules do or do not support various real-world tactics.
For the sake of argument, let's assume little or no magic. Once we understand how combat works without magic, we can add it back into the discussion.
With the existing skirmish-level rules, a mass combat doesn't lead to the expected outcome. Following Sun-Tzu confers little advantage. Attacking up a hill isn't an issue, keeping archers to your shield-side isn't an issue, making good use of standards and banners isn't an issue -- and even flanking isn't an issue.
What does matter in D&D? If we ignore magical artillery, you want a tight formation -- or at least a tight mob -- because it puts more of your soldiers in the same area against fewer of the enemy's soldiers. Real-life formations were often tighter than one man per five feet, more like one man per three feet, and this was an advantage for gladius-wielding legions against broadsword-swinging barbarians -- they could get 2-to-1 odds at the line -- but for D&D, fitting one man in every 5-foot square is good enough.
Flanking, per se, doesn't matter in D&D, but if you can waltz down a line of soldiers, you can achieve overwhelming local odds. Lined up east to west, facing north, his troops can only engage one at at time against a surprise attack from either east or west. Or, oddly enough, if his troops are marching north along a road, you probably want to attack from the north (or south). That's how flanking works with no notion of facing.
Real-life formations made extensive use of large shields. A wall of interlocked shields keeps out most arrows and javelins -- and most melee spear attacks. It only works to the front and left though, which explains why troops would drift to the right -- each guy wanted to get behind his comrade's shield -- and why flanking is so important.
It also explains why breaking the line was so important. Spartan Hoplites trained extensively for the "scrum" between lines of soldiers with large, bronze shields. Knocking back the enemy -- a Bull Rush in D&D terms -- meant breaking his lines, exposing his men to spear points, and sending them running. In D&D, breaking the line means stepping into a Flank Attack, not creating one. Anyway, it's pretty clear that you want a downhill run, not an uphill one, if you're aiming to collide with the enemy lines and break them.