Two unsupplied units entering a non-allied nation's territory. Pikemen, as military units, suffer damage by attrition, indicated by a red glow under the unit. On the other hand, the Explorer marching ahead is a scouting unit, among the few immune to attrition.
Attrition damage is damage inflicted on land units that pass through enemy territory without a Supply Wagon nearby. It appears as a bright red glow beneath a unit.
Attrition is researched at the Tower, and can be upgraded throughout the ages. The base rate is 1 hit point every 3.2 seconds (48 frames). Each upgrade doubles the previous rate.
When damage is inflicted, players will see a blinking red circle around their units indicating their health is being lost. Note that the more attrition damage is being inflicted, the brighter the red circle will be.
Several bonuses can be obtained to increase attrition damage to enemy units, such as wonders (such as the Colosseum and Kremlin), being at a higher age, or choosing the Russian civilization (100% more damage inflicted).
Buildings in enemy territory suffer from attrition as well, taking a fixed 8 points of damage every 2.1 seconds (32 frames) until the structure is destroyed. Supply Wagons cannot protect buildings from attrition and the rate cannot be increased or decreased in any way.
Unit differences
Units made up of 3 sub-units (such as Slingers) take somewhat more attrition damage if all 3 sub-units are under attrition. This does not apply if only 1 or 2 sub-units are remaining. The amount depends on the level of attrition; for example, it's about 17% more for the base tier 1 attrition and about 12% more for tier 2 attrition.
Artillery weapons (such as Catapults) take attrition at half the rate.
The Militia line of units (Militia, Minutemen, and Partisan) take 4x attrition, and do not benefit from the Forage line of technologies at the Smelter, the Statue of Liberty wonder's reduction in attrition, nor a Supply Wagon's protection from attrition.
Countering attrition damage
Players can counter attrition by bringing Supply Wagons or Patriots of totalitarian governments (Despot, Monarch, Comrade) to the battlefield.
Researching Forage, Supply, and Logistics technologies at the Smelter can decrease the basic rate of normal attrition that units sustain from the enemy's territory without supplies. It also allows units to not take attrition damage in enemy territory if they are not moving nor fighting. Also building the Statue of Liberty wonder can reduce attrition damage in hostile territories.
Artillery weapons such as Catapults take half the rate of attrition damage.
The Mongols take half the rate attrition damage. They also receive the Forage line of upgrades for free (once they meet the requirements). Due to this, they normally have the upper hand in invading enemy nations at early ages.
Attrition and Diplomacy
Units within a neutral nation's territory (The units' parent nation is at peace with the nation whose territory is being crossed) will suffer severe attrition damage, regardless of supply units within range, or technologies researched by either nation. The only way to remove the attrition penalty is by effectively terminating the diplomatic neutrality: Either by forming an alliance, as allied nations can cross each other's territory without suffering attrition; or by declaring war, in which case all the common rules of attrition apply.
Immunity to Attrition
Attrition may only affect land units; so ships, aircraft and missiles will never suffer from it. In addition, the following land units are immune to attrition: