I know the battle of Agincourt was famous especially because of the English's heavy use of longbows. Was this because Henry V had a stroke of tactical ingenuity and chose to saturate his ranks with so many archers to great affect, or was this due to circumstance that demanded Henry rely on archers and he just got lucky at how successful they were? I understand that the battle of Agincourt was very unorthodox from what I understand from other Medieval battles, so I'm just curious as to how this famous battle's unorthodox strategy came to be.
The employment of massed archers in English armies predates Agincourt by hundreds of the years. The prevailing understanding among historians is that the effect of the bow was experienced by the Anglo-Normans during their incursions into Wales in the 12th century. Notably, the Assize of Arms in 1252, mandating the ownership of bows by men between 15 and 60, was issued during this time. The effect of bows against the English armies was noted by Gerald of Wales:
[The arrow] went right through his thigh, high up, where it was protected inside and outside the leg by his iron cuirasses and then through the seat of his leather tunic; next it penetrated … the saddle … seat; and finally it lodged in his horse, driving so deep that it killed the animal.
The border conflicts between Wales and the western counties of England would lead to the wider adoption of the bow as a military weapon in England. The understanding is that the English adaption of the longbow, along with improvements in armour, would result in the upscaling of the bow to the large yew staves and heavy draw weights that they are renowned for.
The archer was a staple of English armies (and navies). The Battle of Falkirk in 1298 - the downfall of the Scottish rebellion under William Wallace - demonstrated the devastating impact of massed archers against densely packed infantry formations, to which the Scots had no answer. The finest moment was perhaps Crecy in 1346, in which the English armies proved their superiority over Genoese crossbowmen and then rained havoc onto the French army. Agincourt is the most well documented and romanticised, not least due to Shakespeare's Henry V, but the deadly effect of massed archers had been seen for hundreds of years across battlefields in France and Scotland.
So, why so many archers? The simple answer is that the archer was the most economical choice for the purpose they were recruited. The English campaigns in France - chevauchée - consisted of raiding and pillaging of French towns. English archers were mounted, moving swiftly and dismounting to fight, using their arrows to overwhelm town defenses. Their battlefield application was almost a bonus rather than their primary function. The force at Agincourt was not an army put together to conquer - they only fought because they had no choice but to face the pursuing French army.
Agincourt wasn't a master-stroke that Henry V came up with. It was the result of centuries of experience and success with the bow as a weapon of warfare and, to the French, a weapon of terror. The English tasted this when they faced the Welsh. They trained their armies to use it against the Scots, and the arrows covered the fields of Poiters, Crecy and Agincourt. For as long as the French kept on charging headlong into the English lines, the longbowmen of England would shine.
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