Casus belli
A 'justification for war' — a formal reason a nation must have before declaring war, with mechanical consequences for fighting with or without one. Paradox grand strategy games (Crusader Kings, Europa Universalis) use casus belli to constrain and shape aggression: a valid claim lowers costs and defines war goals, while warring without one incurs penalties (stability hits, coalitions, reputation loss). Designers use it to model diplomacy as a system with friction, to prevent constant frictionless conquest, and to give expansion a narrative and legal texture. Key decisions: how CBs are acquired (claims, culture, religion, fabrication over time), what they permit (limited war goals versus total conquest), the penalty severity for warring without one, and how AI weighs them. Pitfall: if fabricating or acquiring a CB is trivial, the whole constraint is theater; if it's too onerous, expansion stalls and the game grinds — the tension between the two is the tuning challenge.
- Dev effort: Medium
- Timing: Real-time or turn-based
- Common in: grand-strategy, 4x
Seen in
- Crusader Kings III
- Europa Universalis IV