Modern tests and contemporary accounts agree therefore that well-made plate armour could protect against longbows. However this did not necessarily make the longbow ineffective; thousands of longbowmen were deployed in the English victory at Agincourt against plate armoured French knights in 1415. Clifford Rogers has argued that while longbows might not have been able to penetrate steel breastplates at Agincourt they could still penetrate the thinner armour on the limbs. Most of the French knights advanced on foot but, exhausted by walking across wet muddy terrain in heavy armour enduring a "terrifying hail of arrow shot", they were overwhelmed in the melee.
Less heavily armoured soldiers were more vulnerable than knights. For example, enemy crossbowmen were forced to retreat at Crecy when deployed without their protecting pavises. Horses were generally less well protected than the knights themselves; shooting the French knights' horses from the side (where they were less well armoured) is described by contemporary accounts of the Battle of Poitiers, and at Agincourt John Keegan has argued that the main effect of the longbow would have been in injuring the horses of the mounted French knights.