Hundred Years' War

[Brenus]

So, Crécy and Azincourt were not decisive: If the French would have forgotten their civil war, the English would have even more troops in front of them. If confronted to a united French Army (and without rain, in case of Agincourt), the English could claim the victory...
If the German in 1940 would have waited that the French army to be equiped with the new and moderm weapons and material (and the General en Chef to retire) and still would have been able to defeat the French and English etc…

Whatever the political circumstances, a defeat is decisive when you lost what you had. That is what did happen to the English. Even before the formation of the Franc-Archers (1448) the French army captured Creil and Conflans. French besieged Pontoise in June, and took the town (25 October 1441) after a long campaign and an artillery siege directed by Jean Bureau.
In 1442 Charles VII launched a major campaign into Guyenne and captured St.-Sever, Dax, and La Réole.

The defeat (in the final period) for the English at Montargis followed by Patay and then Lagny-sur-Marne is a clear signal that the English started to loose for good this time. Which battle was the decisive one? Yes the English did succeed to hold and even to re-conquered some towns, however the Truce of Tours suspended hostilities between England and France (lasted until 1449). As part of a two-year truce, English surrendered Maine. After the English broke the truth, they went from defeat to defeat, losing town after town.
Formigny (15 April 1450) followed by Castillon (17 July 1453) were just the last battles which finished of the Hundred Years War. They are decisive because the last.

Patay in 1428 (18th of June, quite a lot of battles the 18th of June) was decisive. It save Rouen from the English and is not often mentioned…