Stratagems are devices or "tricks" used to deceive opponents in the course of warfare. Differing stratagems come into play during invasions and in province defense.
Ambush: An ambush is a surprise attack launched from a concealed position. A successful ambush can inflict serious casualties upon opposing troops while your forces sustain minimum losses. A poor ambush (one that is not much of a surprise) can result in total annihilation of the ambushing force. An ambush may only be attempted by troops of a provincial army in defense of the province. An army attempting to ambush suffers a loss to its overall maneuverability.
Patrol: Patrols can gather useful information on the strength of opposing forces prior to a battle. An accurate estimation of the number and kind of troops which your army faces can be a decisive advantage in set piece battle. You may wish to command invasion forces to patrol, while provincial forces defending a province will dispatch patrols automatically. Troops on patrol may return late for any battle which occurs or be drawn into a lone skirmish with enemy troops. Late troops cannot be assigned in the following set piece battle.
Hidden Movement: The counter-strategem to patrolling is hidden movement. Both invasion forces and troops defending a province may attempt hidden movement. An army attempting hidden movement will employ any number of tactics designed to confuse and distort the information enemy patrols might gather. This may consist of moving through concealing terrain, marching only at night, or driving animals before them to raise dust and create the appearance of a larger army. An army attempting hidden movement sacrifices a great deal of its overall maneuverability. However, successful use of this strategem may equally impair the maneuverability of enemy forces.
Decline Battle: An army may attempt to avoid conflict with enemy forces
altogether or decline battle using this order. The chance of successfully
declining battle is exceedingly slim unless both commanders are attempting
to do so at once. Even then, the possibility that the armies will not engage
is no better than 50/50. An army attempting to decline battle sacrifices most
of its offensive maneuverability to increase its chances of avoiding the
enemy's offensive efforts.