Recruiting, Organization, Tactics
The experience of war must inevltably change the practice of war. And in a war lasting more than a century, the practice of war hanged enormously. Of course, from the perspective of the 20th Century -- when the ways of war undergo almost daily change-- the evolution of the military art during the Hundred Years' War seems minor. Yet there were changes, and they were both significant and of far reaching implications.
Recruiting
At the start of the wars, ln 1337, the theoretical basis for military service in both France and England was still feudal obligation: a person owed service as a consequence of holding lands from the King, or from someone who held lands from the King. A particularly prosperous individual might owe a great many men-at-arms and infantrymen. A relatively impoverished person might be obliged to club together with several others of his ilk to supply but one man-at-arms or infantryman. However, if this remained the theoretical basls of military service it was by no means practical under most circumstances, such as foreign conquest. As a result it had been virtually abandoned in England.
For generations England had been involved in wars of conquest against Ireland, Scotland, Wales and, of course, France. In these wars the traditional obligation of 40 days, which worked fine for home defense, proved impractical. A campaign would barely begln before the troops' obligation had expired. So a practice called scutage had been invented. A person owing militiary service paid a small fee to the King and was rid of his --or her-- obligations for that year. Since some people did not find military service particularly onerous, the king could then contract --"indent"-- with them for regular long-term service on the royal payroll, using the proceeds from scutage to pay them. Thus England developed somethlng of a regular army , consisting of hundreds of private contractors willing to provide service anywhere and anytime at the King' s pleasure and expense. It was these men who formed the backbone of the English armies through the Hundred Years' War.
Now scutage existed under French law as well, but France was so large --as was the military class-- that even by relying upon feudal obligation it was usually never difficult to collect a large army. The King could always find some money to keep some men active if he really needed them beyond the traditional obligation. So the armies which met the English at Crecy, Poitiers, Agincourt and on many another occasion essentially consisted of feudal levies; masses of reservists with a small leavening of professionals. Many of these people were not particularly skilfull, nor experienced, and many had their heads full or chivalrous nonesense. And since they were not necessarily on the royal payroll, they were not necessarily going to obey orders.
Gradually the French improved, though not without painful disasters. The success of DuGuesclin later in the 14th century was partially due to the establishment of something akin to the English indent system. Of course the introduction of irregular warfare and gunpowder helped a great deal, but --aside from an increasingly favorable political situation-- the real breakthrough for the French came in 1445 with the organizatinn of the Compagnies de l'Ordonnance, the first organized, full-time, salaried standing army in Europe since the fall of Rome. This gave the French a technical superiority over the Engllsh, proving a significant factor in French successes during the final phase of the war, in 1449-1453.
Organization.
There were surprisingly little organizational differences between the two armies. Indeed, there was little variation af any sort through the wars as a whole. Field forces were composed essentially of cavalry and infantry.
The cavalry was mostly heavy cavalry-men-at-arms (Knights and "serjeants ") with a small contingent of light horse. These were formed into "lances," basic combat teams comprising two or three armored men (normally a knight or serjeant, one or two squires ) and usually one or two lightly equipped mounted troops (pages or bowmen). Several lances formed a "post" under the senior man-at-arms, and several posts made a "banner" frequently under a veteran knight called a "banneret." Even the French compagnies de l'ordonnance were organized on this basis, each company really comprising a single banner.
The foot soldiery usually consisted of archers and several types of melee infantry. As time went by the archers --whether longbowmen, shortbowmen, or crossbowmen-- increased in numbers at the expense of the other types of foot (spearmen, javelinmen, daggermen, axemen). In the English service, and probably also in the French, the foot was formed into platoons of 20 men under a double-pay man (a "ventenar," or "twentier." ) Five such platoons would comprise the command of a"centenar" ("hundreder"), usually a squire, who received even more pay. Several of those went to form a battalion, and we hear of specific commands of a thousand men during the 13th century. These were often commanded by experienced knights, who received a knights pay , and usually a little more, for the responsibility required to lead so many troops.
Finally, of course, there was the artillery. Both the English and the French had mechanical artillery , various types of catapults and trebuchets which could be surprisingly effective. But chemical artillery was just coming into existence during the Hundred Years War: There is some slight evidence that there may have been some primitive cannon at Crecy, while the French certainly had several at Agincourt, albeit they accounted for only one Englishman killed. The French took the lead in the development of artillery, if only because they spent most of the war on the losing side. Much of their success under DuGuesclin and in the closing years of the war were due to their possession of superior artillery: Certainly the French cannon were the decisive factor in their victory at Formigny.
Tactics
Through most of the Hundred Year's War the English were the tactical masters. Their Welch, Scottish, and Irish wars had given then a lot of unique combat experience which served them well in France. Their basic tactic was quite simple. The always tried to fight on the defensive, using dismounted men-at-arms to form a phalanx, behind and to the flanks of which were formed masses of archers, all covered in front by wolf pits or sharpened stakes, and with their flanks resting on secure natural obstacles. Such a defensive posture would force an attacking foe --whether ahorse or afoot-- to come at them from the front. In the process, the attackers would suffer greatly from the enormous volume of arrow fire.
Robin Hood to the contrary notwithstanding, it was the volume --not the accuracy-- of the arrow barrage which would cause the attackers problems. Consider that 6,000 archers carried a quarter of a million arrows and could easily discharge them in a fairly leisurely fashion in a hour or so. In a hurry they could theoretically get them off in a couple of minutes, with devastaging effect on any attacker. Thus, by the time the attackers were able to come to trade blows with the dismounted English men-at-arms they were already pretty cut up, and possibly very disorganized. Of course, in order for this system to work properly the enemy had to do the attacking. Usually this was no problem, as the French tended to be overly agressive. On those occassions when they were less so, a few flights of arrows at long range usually stung their honor sufficiently to force the issue.
The French ought to have been able to cope with the English tactics with relative ease. They had, in fact, some experience combating steady infantry, having been thoroughly waxed by Flemish citizen-infantry at Coutrai in 1302. But they had attributed their defeat to the boggy nature of the ground, which was only partially correct. Moreover, they had several times over the following 20 years inflicted severe reverses on the sturdy Flemish burgers, and failed to understand the reasons for their victories, which were just as easily explainable by citing unique circumstances as had their reverse in 1302. Worse still, there was little of what has come to be called "institutional memory" among French military leaders, who were themselves little more than reservists as well.
So the French continued to believe that a head-on clash was the key to victory, even when one was unnecessary. At Crecy they tried it mounted. Blaming their failure there on the English trick of dismounting their men-at-arms, the French tried dismounted combat at Poitiers and again at Agincourt. They never caught on to the idea that the English technique worked only in defense. Never, that is, until Formigny, when their two cannon gave them the chance to force the English into making an attack, which was essentially what the English had been doing to them for over a century.
Strategy
It is sometimes difficult to discern whether some leaders during the Hundred Year's War had a strategic vision. For some it is probably true that strategy was little more than pointing an army in a particular direction and turning it loose. But clearly men like Edward III, the Black Prince, Bertrand DuGuesclin, and Henry V, had a clearer notion of strategy. Operations were often undertaken with their political implications in mind, such as Henry V's campaign in Normandy, which would simultaneously reclaim a province historically in the possession of the Kings of England and threaten Paris itself. And Henry's enforcement of rigid --even draconian-- discipline with regard to the behavior of his troops strongly suggests that he had a clear idea that winning ultmiately meant securing the "minds and hearts" of the Normans.
But in the end, the English could never figure out how to win the war, although Henry V came close through dividing the French. The French, of course, could see that the way to win was to clear the English out of France. But it took them a long time to learn how to win the battles which would permit them to do this. Ultimately, of course, France was transformed on the anvil of the war from a collection of petty feudatories into a nation. Once that happened the English cause was doomed.