If your kingdom is attacked by an army, it will make a Stability check to defend itself.
If there is a defensive improvement in the attacked hex, or in an adjacent hex, your kingdom adds the Defence bonus to the stability check.
You can make a Loyalty check to temporarily increase the kingdom's Recruitment laws, which will increase Stability at the cost of Upkeep. You can also spend BP to temporarily outfit your forces with special gear, which will may give a larger bonus or otherwise affect the outcome of battle (e.g. giving the entire milita silversheen before facing a werewolf army).
If the player characters can reach the battle in time, you can engage in an encounter against some of the enemy forces. If you are successful, your kingdom gains a bonus to the stability check. You can choose what difficulty of encounter to engage in; the more difficult the encounter, the larger the bonus.
If you want to wage war beyond the borders of your kingdom, you must raise an army. This will require a successful Loyalty check, and a regular payment of BP.
Once you have an army, you can manoeuvre them to attack enemy armies or locations. This will require Stability checks, with a bonus based on the size and quality of the army. As with defensive war, players can chose to engage in a combat to give the army a bonus to its check.
If your army are in the field for a long time, or are defeated, you will have to make Loyalty checks to avoid gaining Unrest.