Each kingdom has one imperial army for every three provinces it controls rounded to the nearest whole number. Each kingdom has a minimum of two imperial armies (exception--when the government is in exile). The number of imperial armies controlled by each kingdom is assessed at the end of each turn and armies are either lost or gained based on the number of provinces a kingdom controls at the end of the turn. Troops assigned to imperial armies which are eliminated due to a reduction in the number of provinces a kingdom controls are automatically sent to the capital province in order to await re- assignment. New imperial armies created due to an increase in the number of provinces a kingdom controls at the end of the turn are automatically created at the capital province with 0 troops initially assigned. This 0 troop or "bookkeeping army" can be moved up to 12 provinces per turn until such time as troops assigned to the army from the capital have arrived and are listed as on duty with the army in question. At such time the movement rate of the army is reduced to that of the slowest troop unit in the army.
Unlike provincial armies, imperial armies not only defend the province they are assigned to, but may also be placed on a special "defensive status" that allows them to respond to attacks on neighboring provinces. The status of an imperial army may be changed from active to defensive and visa versa. An army changing its status will undergo a one turn transition period during which, in addition to the standard limitations of its original status, it will only fight in battles occuring in the province where it is located, and may not itself initiate combat.
An imperial army on defensive status can respond to attacks on provinces several provinces away. Since different troop units have different movement capabilities, the response to such attacks is made by individual troop units. For instance, a light cavalry unit may be able to respond to a distant attack, but a heavier cavalry unit could not travel such distances as quickly. Also, that same light cavalry unit could respond to more than one attack in a given war season. The response movement ability of a defensive troop unit is its movement ability minus two. (A troop will always have a minimum response ability of 1.)
Even though troops from a defensive imperial army may respond to attacks outside their assigned province, the army as a whole does (and must) stay in one province, and may neither raid or invade until placed on active status. troops stationed with defensive imperial armies will only respond to enemy Invasions - they will never respond to an enemy raid. Only provincial armies respond to enemy raids. Defensive imperial armies are significantly less expensive to maintain than those on active status.
Raid: An imperial army on active status has two primary purposes. The first of these is the raid. Active imperial armies with eight or fewer troop units may raid; those larger than eight troop units in size may not. Troops may not detach from an imperial army to raid as they may from provincial armies. If you assign an imperial army with more than eight troops in it to raid, the raid will not take place.
Invasion: Active imperial armies may move freely through your provinces as
well as those of your formal allies up to the maximum movement ability of
their slowest troop unit. They are the primary forces of invasion and at
least one active imperial army must participate in any invasion which takes
place. During war seasons they may invade any province adjacent to them at
the start of the turn, assuming that proper war preparations have been made.