The war began in a legal argument. When the direct Capetian line failed, the French magnates passed over Edward III of England, whose claim came through his mother, in favour of Philip of Valois, and in 1337 Edward answered by claiming the French crown outright. What followed was less a war than a century of intermittent campaigns, punctuated by the English victories at Crecy in 1346, Poitiers in 1356 and Agincourt in 1415, by long truces, and by the plague that killed more people than the fighting ever did.
The church was inside the quarrel from the start. The popes had moved to Avignon in 1309 and sat, for the whole first phase of the war, in a city on the edge of the French sphere, so English opinion treated papal mediation as French diplomacy under another name and the parliament of the 1350s legislated against papal appointments and appeals. When the Western Schism opened in 1378 the obediences fell out along the war's lines, France and its allies for Avignon, England and its allies for Rome, and Christendom spent forty years with two and then three claimants to Peter's chair while the armies moved below.
Both crowns dressed the fighting in holy language, each side's preachers assuring it that God favoured its cause, and Jonathan Sumption's narrative history traces how thoroughly the machinery of religion, from sermons to processions to the taxation of the clergy, was harnessed to the war effort in both kingdoms.