'Crusade' comes from Latin 'crux' (cross) — named for the crosses worn by Holy Land warriors.
A medieval military expedition to recover the Holy Land from Muslim control; any vigorous campaign for a cause.
From Spanish 'cruzada' and French 'croisade,' both from the respective words for 'cross' — Spanish 'cruz,' French 'croix' — from Latin 'crux, crucis' (cross). Crusaders wore or sewed crosses on their garments, and the expeditions were literally 'crossings' — marked by the cross. English had earlier used 'croiserie' from French; 'crusade' replaced it, combining Spanish and French influences. The metaphorical sense of any zealous campaign dates from the eighteenth century. Key
German 'Kreuzzug' literally translates 'crusade' as 'cross-pull' or 'cross-march,' making the etymology transparent. The English word, by contrast, hides the 'cross' inside a Spanish/French blend that obscures the connection. The word 'crucial' is also from Latin 'crux' — something crucial is 'at the cross' or crossroads of a decision.