Light infantry dodging everything

Loose foot and loose mounted troops have the ability, at their option, to withdraw from ranged or melee combat, thereby reducing damage by the amount of hexes the move away (at least that is what the game seemed to imply, it wasn’t very clear) up to their full marching movement.

My question is, what stops a careful, fast-moving loose-mounted unit (such as light infantry with marching movement of 4, iirc) from being near invincible while peppering the flanks of the enemy?

Hey there! Great question.

First, historically, a careful, fast-moving loose-mounted light infantry unit could destroy a heavy infantry unit by peppering it with hit-and-run attacks. The Battle of Lechaeum is a famous instance of this, one that that taught the Greeks that they had to increase the proportion of light infantry in their armies.
See http://en.wikipedia.org/wiki/Battle_of_Lechaeum

Many battles featuring heavy troops against horse archers demonstrated similar outcomes.

In terms of game mechanics, several factors make it difficult for a unit to actually do this.

  1. Units cannot withdraw if they are disordered. A unit which withdraws becomes disordered. Therefore you can never withdraw from two consecutive attacks.
  2. Units can only withdraw up to their marching movement. For light infantry, that is only 2 hexes. Heavy infantry can charge 3-4 hexes. For horse archers, that is 4 hexes. Sword-armed heavy infantry can charge 4 hexes, and heavy cavalry can charge 9 hexes. Thus it is impossible to withdraw outside of charge range without having blocking troops.
  3. A heavy unit under fire from a light unit can defend in lieu of attacking (+2 to +4 AC), giving it a likely AC of 6 to 10. The light unit will hit on between 5% and 20% of its attacks, while depleting itself of ammo on 5% of its attacks. Before it destroys the heavy unit, it will likely run out of ammo.

So if you’re playing a heavy force against a light force, you have two basic strategies. Either you can hunker down, wait for the enemy to get depleted, and then fight them in melee (where they will lose); or you can attempt to whittle down their force by disordering them, and hitting them again while disordered, either with good troop placement or lucky initiative rolls.

Slightly on the subject, there’s a guy named Dan Carlin who does a podcast called Hardcore History. I really enjoy them, and one of the most recent series he did was on Genghis Khan. He talked quite a bit about the Steppe horse archers and their brutally effective tactics against the Middle Eastern kingdoms and the armored knights and heavy infantry of Europe. If you like that sort of thing and have some time it’s a fun listen.