nah thats bull.
Very few blades can cut through plate (falcions mostly), the thinness of that blade weilded by an average man, would do bugger all to plate. Probly wouldnt even knock the dude of balance.
Heres a do it at home experiment you can try on armour piercing, get your steel frying pan, grasp your bread knife and stab your pan. That is the equivalnt to a thin blade and plate armour. Anything happen to the pan?
The strenght of plate is one of the reasons pole arms (warhammers, pole axes, glaives, halberds ect) became so used, they cut through plate by buckling the metal and blundening the poor chaps to death.
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And on the longbow it is over hyped but it is still deadly as **** (i am a archer, an can pull a 80lb longbow i know how powerfull it is, and thats not even a warbow, just sold my 60lb bow, to france of all places??) But it could kill and did kill even knyighits in plate didnt go striaght through them, thats teh era when chain was being replaced with plate when it was most leathal. Still couldnt go striaght through an armoured man though, unarmoured yes.
And even if it couldnt pierce right through you try having 3 arrows even piercing 2 inches into you, you wont be fighting and if it didnt pierce it could buckle and knock the guys over or off their horses. And 300o guys losing so many arrows a minute at you and your fellow knyinghits will result in huge casulites, but maybe not many instant deaths.