You say need. I don't really see why that would necessarily make for a better game at all. Perhaps you could sketch out some specifics as to how you think it would improve combat?
Full disclosure. I think greater complexity in the combat system could well end up trying to turn EU IV into a different sort of game, and leave it satisfying noone much. Complexity for the sake of complexity isn't a good thing in my opinion. Particularly given the only complaint about shattered retreat seems to be that it is unrealistic, rather than gameplay problems it has.
I agree that we would not want complexity just for the sake of complexity.
Up front, two points to emphasize for players and Paradox:
- The Withdrawal (Under Fire or not) is still a part of Battle in all military training and lexicon. It is not a separate thing from Battle, it is in a final phase of battle.
- The maneuver referred to as "Withdrawal Under Fire" is a commonly recognized phrase among all Western militaries for centuries, and this maneuver is considered the absolute #1 most difficult tactic to train for military officers and soldiers. In many cases, worldwide, even in modern day, it is far too ignored due to over-confidence in a military force that assumes it will not lose a battle, and will not have to perform this maneuver, so it is both the most difficult, yet least trained tactic, on average, among all modern military forces.
- A Retrograde is a form of Withdrawal that has a plan to counter-attack in leap-frog staggering maneuvers, but I am not discussing that option in this note (but it is yet another option that could be added to Paradox's EU series or otherwise).
A "how to" - to keep it simple and yet better integrate what is a Withdrawal or Withdrawal Under Fire into this game, first with a Macro Concept:
- a Withdrawal allows a more distant retreat from the province (perhaps 2-5 provinces away, at most, depending on stats variance from enemy), whereas a Withdrawal Under Fire should only allow a 1-province distance for retreat (you are slowed as you are "Suppressed" by fire and/or maneuver forces causing attrition in your Withdrawal Under Fire).
Another set of factors that helps show change, from a Multiplayer point of view --
- Example 1: Player one's Army + Commander has better speed/Maneuver but will lose the main phase of battle (what we see play out in small pop-up with dice rolls and one side loses more forces and/or morale than the other), while Player two has the upper hand to win the main-phase battle but not as much speed/maneuver, so then a pop-up comes up for Player 1 or a self-selection button is available for "Withdraw" (not Retreat) as that would not be under fire, because the better speed/maneuver cavalry force would outpace the opposition. There should be a cost, yet more % lost as you Withdraw, that an algorithm could factor more/less depending on a not-in-current-game stat of forces and/or commanders to reflect their ability to disengage from combat, but in simple terms, one could expect a smaller percentage loss in disengagement from forces that are superior in speed/maneuver.
- Example 2: Opposite stats but same battle result, Player 1 is lower speed/maneuver for Army + Commander AND will lose main phase of battle, in contrast to Player 2 who is superior in the main phase of battle and will win, AND has superior speed/maneuver and will outpace the slower Player who tries to Withdraw. So, Player 1 would have an algorithm that forces him to choose "Withdraw Under Fire" and only that 1-Province retreat, with a greater percentage of forces lost during the Withdrawal Under Fire in contrast to Example 1. The Dev's could leave in small chances in algorithm to enable even this scenario to have a 10% chance to still receive a "Withdraw" move, that then allows the 2-5 province distance retreat, but it should not be the norm, on averages.
- Example 3: Near-equal stats for speed/maneuver and combat power (Army+Commander), then you get an algorithm working that factors out your chances to Withdraw vs Withdraw Under Fire, with RNG Luck being on your side if you get the preferred chance to Withdraw. I wouldn't call it 50/50 odds, there could be other factors such as who started on Offense vs Defense, what the proportion of remaining forces are for Player 1 vs 2, etc, so it should be an algorithm that factors your lottery odds.
This would move us into a more realistic Withdraw vs Withdrawal Under Fire system at the last phase of battle, while still parsing out that small popup main phase of battle that precedes it, even if the remainder of core battle mechanics were left untouched.
Take my Multiplayer description and make it computer/RNG for player 1 or 2, and it's the same thing in Single Person play, but not interaction (in Multiplayer, it could be set up that one of the players chooses to allow Withdrawal of their opponent, out of kindness).
Last thing I have thought of - the Withdrawal Under Fire should have some attrition cost to the Pursuing forces also, and it could be countered in some rare cases depending on some special skills for Counter-Attacking, etc., but pursuing an enemy who is Withdrawal Under Fire should still have some cost to it, and the Player 2 should get an option to cancel the Pursuit if that player thinks any further losses or risk is not worth the Pursuit.