In Warhammer 40,000, the galaxy’s great factions usually have a very serious reason for going to war. The Imperium fights to survive, the Aeldari try to stave off extinction, the Necrons want to restore their empire, and the forces of Chaos pursue ambitions of their own. Even the Tyranids, alien as they are to most other civilizations’ concerns, obey an implacable logic: find biomass and devour it.

Among the Ultramarines, war must be understood before it is fought. Strengths are listed, objectives defined, risks classified. Varro Tigurius is the man responsible for studying that which precisely refuses any classification: the Warp, the visions and the thoughts of enemies whose minds have nothing human.

Roboute Guilliman is often summed up in a joke: a Primarch capable of conquering the galaxy with an Excel spreadsheet! The caricature works because it contains an element of truth. Guilliman plans, classifies, delegates, measures and builds. But to reduce the Lord of Ultramar to a superhuman bureaucrat is to forget what organization means in Warhammer 40,000. In the midst of an empire that readily confuses suffering and virtue, making a road passable, feeding a planet and tra