Warfare in EU5: Casus Belli, Warscore, and Why Fair Fights Lose

EU5 wars are decided before the armies meet, in three systems most new players read too late: the casus belli that prices the war, the warscore that defines winning, and the exhaustion that taxes every month you stay in it. This page is the machinery; the doctrine — fight only downhill — runs through every nation guide on this site because the machinery below is why it’s true.

Two kinds of war

Country wars are the normal case: declared with (or without) a casus belli, fought over occupations, settled by treaty under warscore rules.

Civil wars play by older, crueler rules: they fire when a rebellion reaches full progress, use no CB, no warscore, and no treaties — locations change ownership as they fall, and the war ends only in someone’s complete annexation. This is why our guides treat estate satisfaction as a military statistic: the civil war you prevent is the only one you’re guaranteed to win.

The casus belli economy

A CB is a priced justification. Most are fabricated for 20 Spy Network and expire 4 years after creation if unused; the better ones arrive free — from situations (the Ottoman opening lives on the Rise of the Turks’ Press Claims), international organizations, parliament sessions, and events. Each CB fixes the war’s goal, and the goal is not decoration: holding it ticks warscore up over time, failing it ticks warscore down, and the ticking impact resets when the goal’s status changes.

Declaring without a CB is legal and expensive: a generic Superiority goal, +50% conquest cost (+30% subjugation) at the peace table. Declaring on a friend costs on a second axis — +10 stability against a country at 50+ opinion, +20 at 100+. The Byzantium no-CB-Ottomans debate in our guide is exactly a debate about whether removing a future superpower is worth paying all of these at once.

What warscore is made of

Warscore comes from battles, sieges, blockades, and the wargoal — and battle warscore is capped at 50%. That cap is the single most important sentence in EU5 warfare: you cannot fight your way to a dictated peace. Sieges and the goal do the second half.

At the peace table, every location carries a war-worth assembled from its attributes: a base value, premiums for having a port, being a province capital, being a country capital, scaling per fort level and per development, plus population and building components up to caps. (The exact weights live in game-file defines the wiki references without displaying; we’ll publish the numbers when our file-level verification lands rather than guess them here.) The design consequence is what community campaign write-ups report from the field: rich, forted capitals can exceed 100 warscore alone. Wars in EU5 buy specific things, not maps.

War exhaustion: the meter that ends wars

Every point of war exhaustion costs, concretely: −2% army morale, −2% navy morale, −1% production efficiency, −1% trade efficiency, −0.03% population growth. It decays at 0.1 per month — at peace. Run the arithmetic once and short wars stop being a style preference: ten points is a decade-scale economic hangover, and a country pushed high enough is forced to the table regardless of the map. (Yes, the console’s WarExhaustion command accepts negative numbers; no, your campaign shouldn’t depend on it.)

Combat: the dice are loaded before the roll

Battles resolve in repeating phases; each phase both sides roll a six-sided die that sets their damage until the next roll. Everything strategic happens to that die and to the frontage around it:

• Terrain stacks. Mountains give an attacker −2 dice and woods −1 — a mountainous-woods location is −3 before anyone swings, and the same terrain cuts frontage for both sides (−4 and −2 respectively in that example).
• Crossings punish. Attacking over a river or through a strait adds further dice penalties (define-referenced; the community shorthand — never cross into a prepared enemy — predates the exact numbers and survives them).
• Units care about ground. Type modifiers are percentage damage swings: galleys +50% in narrows, cavalry −10% in wetlands, mountains and jungles, and unique units go further — Irish Hobelars take +50% in woods and hills.
• Artillery opens. If either army fields artillery, a five-hour Bombard Phase precedes normal combat, each hour giving artillery a bombardment-efficiency chance to damage an opposing regiment.
• Damage is mass times quality. A regiment’s damage is its combat power multiplied by current strength; ships deal by cannons and absorb by hull size. Half-strength armies hit half as hard — reinforce before you re-engage, not after.

The doctrine our nation guides repeat — smaller enemies, better ground, their river not yours, sieges you can afford — isn’t caution. It’s just this arithmetic, read in advance.

Common death spirals