Units with active defense ability will automatically fire once during opponents turn at the first enemy unit that comes within one tile range. This attack works exactly like <l:6,19>Bombardment<e> attack of the unit in question, but will be executed regardless of any considerations, like having moved, assaulted or bombarded last turn. Units which have Active Defense but lack ordinary Bombardment (like <l:1,34>Mobile SAM<e>) are considered too clumsy to actually chase their prey.

- There are four types of Active Defense: Land, Sea, Air and Space. Unit of a different type than stated in the description of the active defender won't trigger the Active Defense (for example, a <l:1,131>Fighter<e> won't trigger <l:1,31>Artillery's<e> Active Defense).
- Some units won't trigger Active Defense despite matching type. On land, only units that have both Land and Mountain movement will trigger Active Defense. Others are either too swift (like cavalry or tanks) or prefer long-range attacks (like artillery pieces). On Sea, units that cannot enter deep oceans won't trigger Active Defense, as they're keeping low profile.
- A unit must be spotted first to trigger Active Defense, but the spotting doesn't have to be done by the firing unit.
- Active Defense is symultaneous, not sequential. When a unit triggers Active Defense, it triggers Active Defense of any and all defenders within the range of their Active Defense, even if it's enough to kill it several times over.
- Active Defense is mutual. If a unit that has Active Defense has been shot at by another active defender, it will retaliate (if normally able to target that other unit with its Active Defense, like for example <l:1,133>Interceptor<e> fighting against another Interceptor).
- Active Defense might cause collateral damage. When active defenders fire upon a stack, all the units within the stack are in danger of being hit, no matter their movement type. Of course, if the active defender's Ranged Attack is unable to harm some of those units (like the Artillery against Fighters), they won't be harmed.