The defense of a country is taken care of by several weapon systems. Each of these defensive weapons is effective against some of the offensive and strategic weapons. It must be part of the unit or garrison that is attacked if you want it to participate in the defense.
Not all weapons are participating in the defense. The participation of weapon systems depends on their distance from the attacked target. The same is true for the federation air force that is called on to help in the defense.
Some of the defensive weapons are used to attack missile batteries in the attacking country. Defense helicopter units and defensive missile batteries (as a part of defensive divisions) are effective in destroying attacking weapons systems. Interceptor units attack incoming bombers and fighters.
When an air wing with interceptors is called to participate in the defense, it will shoot down airplanes that are attacking the target. The interceptors in the defense wing must be in range. The distance we measure is between the place where the interceptors came from and the target that is under attack.
If the interceptors are part of a defense wing that belongs to the country that is under attack, the range is considered zero. If the attack is directed against a military unit that is outside your country, the range is between the air base where the interceptors come from and the military unit that is under attack. If the defending interceptors come from an air wing that belongs to a federation member, the war engine looks at the distance from that federation member to the target.
However, a defense helicopter wing that is called to support the defense is used to attack the attacking unit and its missile batteries at the source. This is the country (or navy) that is attacking the target. The defense helicopter wing must be in range of the source of the attack. The distance is measured between the place where the helicopter came from and the military unit that launched the attack.
The same applies to defensive units and defensive navy missiles that are used in the defense. The distance is measured, between the attacked target where the defensive unit is stationed and the place where the attack comes from (the source of the attack).
Long-range units with missile batteries operating from a long distance will be immune to defensive weapons and must be destroyed by use of offensive weapons that have sufficient range and are effective against these missile batteries.
The defense must rely also on missile interceptors that will reduce the effect of such attacks and on offensive weapons that are used to destroy attacking forces.
Not all defensive weapons are effective against all offensive weapons and they will only participate in the defense if they can be effective against some of the attacking weapons.
Example: Anti aircraft missile batteries will participate in the defense if attack helicopters are used but will not participate if the attacker uses only heavy tanks.
If a target is attacked, the garrison that resides at the target will participate. Some of the weapons in the garrison may be effective and some may not be effective against the attacking forces. It depends on what weapons are used. More details will follow.
In addition, the air force will participate too. If you do not have a defensive radar plane, the air force will not know where the attack will hit. The air force wing that is involved in the defense may send its airplanes to a wrong place and the target will not be defended. If you do have one or more defensive radar planes, the defending air force wing will defend the attacked target.
If you are a member of a federation and members have committed a percentage of their defense forces, then that percentage will participate in the defense if their weapons are within range of the attacker. Losses will be divided between the air force wings of the participants. However, only one air wing that belongs to a federation member will support the defense. The choice will be for the strongest air wing within range.
The best-equipped air wing will be sent for the defense. If the unit suffers losses, the next attack may trigger a different air force unit.
In case there are mobile units in the country and some of the mobile units are within range of the attacked target, the best equipped mobile unit will participate in the defense. This feature is added by the end of Feb 2013.