There are many different combat modifiers in Hearts of Iron 3. Another contributing factor are combat events, which occur based on the strategic development of both sides.
The unit's experience gives a bonus. Many units are produced with 30% experience which will give them a 30% bonus in attack and defense. Units lose experience when they have to reinforce.
This is based on the skill of the leader which ranges from 1 to 5, giving a bonus from between 5% and 30%.
Adds 10% to Leader Skill when defending. So if the leader normally gives a 15% bonus to the unit, with this skill he will give 25% bonus when defending.
Adds 10% to Leader Skill when attacking. See above.
The combined arms bonus is reserved for any with softness between 33% and 66%. When building a division, if the softness icon, at the right of the screen near the bottom of the stats table, has a yellow glow around it, then that division will get the combined arms bonus.
Lack of SuppliesEdit
This penalty gives -10% to the defender for every province they are being attacked from in addition to the first. So for example, if France is beating up an over-confident German army and has surrounded one of their provinces full of units and are attacking from 3 provinces, the Germans will be getting a -20% penalty.
Multiple Combat PenaltyEdit
This occurs when a unit is both attacking and defending at the same time. This can occur when this unit has started an attack and is later attacked while still attacking. This confers a whopping 50% penalty.
This modifier occurs when there are naval units in an adjacent square which are capable of shore bombardment, such as battleships.
The more units there are in a tile, the higher this penalty will tend to be. More highly skilled leaders may alleviate this problem, as well as some research.
Attackers get a 50% penalty during the night. Night equipment can alleviate this to some degree.
Mission Efficiency is for things like bombings and air intercepts. This bonus may be increased by research and can be the difference between a successful bombing run and a complete failure.
This is a negative value based on the dissent score of the unit's nation.