Medieval wars were sometimes won with battles, pitched battles were very decisive in the middle ages. However, they didn't happen very often for various reasons. CKII is very unrealistic in these regards, since combatants will keep raising new forces even after a decisive blow has been struck, two armies occupying the same province will inevitably clash and every single siege either lasts many months or ends in a bloody assault.
This is mainly due to the AI, rather than any deeper issue with the combat. A player understands that their army is necessary in order for them to maintain power and won't risk throwing it all away just to defend a single county, and a player understands that besieged castles are basically already occupied if they have no army left to drive the enemy out. When a player loses a significant battle they prefer to cut their losses early and surrender, unless they're trying to repel an attack that will leave them landless. The AI does not understand this, and will treat every single war as if they're the Soviets fighting the Nazis. It's also problematic that there is no way to negotiate the surrender of a besieged fortress early, and no way to actually avoid combat with enemy armies in the same province.
At the moment, siege assaults (and to an extent sieges in general) are very costly affairs to be avoided at all costs, whereas battles are a great low-risk way to gain warscore. It should really be the other way around. Sieges should be frequent and usually relatively bloodless, assaults should be much less costly if the attacker significantly outnumbers the defenders, sieges should count for war-score even if the holding in question is not yet taken and actual clashes between armies need to be very risky affairs that can change the entire outcome of the war. In CKII, armies are the source of all political power, it's impossible to rule without loyal soldiers to back you up, and the rules of warfare need to recognise that.
This is mainly due to the AI, rather than any deeper issue with the combat. A player understands that their army is necessary in order for them to maintain power and won't risk throwing it all away just to defend a single county, and a player understands that besieged castles are basically already occupied if they have no army left to drive the enemy out. When a player loses a significant battle they prefer to cut their losses early and surrender, unless they're trying to repel an attack that will leave them landless. The AI does not understand this, and will treat every single war as if they're the Soviets fighting the Nazis. It's also problematic that there is no way to negotiate the surrender of a besieged fortress early, and no way to actually avoid combat with enemy armies in the same province.
At the moment, siege assaults (and to an extent sieges in general) are very costly affairs to be avoided at all costs, whereas battles are a great low-risk way to gain warscore. It should really be the other way around. Sieges should be frequent and usually relatively bloodless, assaults should be much less costly if the attacker significantly outnumbers the defenders, sieges should count for war-score even if the holding in question is not yet taken and actual clashes between armies need to be very risky affairs that can change the entire outcome of the war. In CKII, armies are the source of all political power, it's impossible to rule without loyal soldiers to back you up, and the rules of warfare need to recognise that.
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