Combat Modifiers

Combat efficiency modifiers

This is a not entirely compete list of what affects combat efficiency. It is a list of information I picked up while playing the game. If you have more or better information on this please add.

Base efficiency is 100. An A means that the modifier applies to the attacker, a D to the defender

Topology (A)	
 Mountain                    -40
 Hills                       -20
 Broken lands                -10
 Marshes                       0
Vegetation (A)	
 Woodland                    -15
 Forest                      -30
 Clear                         0
 Frozen                        ?     
 Steppe                        0
 Desert                        0
Climate (A and D)            -101
Entrenchment (A)             -1*(average level of entrenchment)2
Fortification (A)            -10*(fort level)
Infrastucture (D)            +20 (if a RR is present, no matter the level of RR)
Landing (A)                  -20 (attacking from a ship)
Encircled (D)                -20 (no friendly province to which the unit can retreat)
Enveloped (D)                -10*(n-1) (where n is the number of sides from which the unit is attacked)
Artillery (A or D)           +SQRT[ART(D)-ART(A)] (the bonus is given to the side with the most arty brigades)
Experience (A and D)         +[unit experience]-10
Gas: one side (A or D)       +503
Gas: both sides (A and D)    +203
Shore bombardment (A or D)   -10*SUM[shorebombardement values of enemy ships]
Stormtrooper bonus (A and D) +1204
Tank bonus (A and D)         +150 (only applies to the unit with tank attachment I believe)4

1. I don't know to what climate is related, it seems to usually show up in the tropics.
2. If a province with 4 division entreched at 100 and 3 at 20 the attacker will get an entrenchment malus of ((100*4+20*3)/7)=65.71. Would this battle be inforced by 10 divisions the entrenchment malus would be ((100*4+20*3+10*0)/17)=27.06
3. If one side has had the two gas inventions, gas attack and gas defence, that side gets a +50 bonus. If both sides have both inventions both sides get a +20 bonus. I'm unsure what happens if you only have the attack capability.
4. The stormtrooper bonus (+120%) and tank bonus (150%) show up after certain inventions. It's not sure what their function is, some say it is to negate the effect of entrenchment.