While watching the Netflix movie The King, in which the prince who would become Henry V engages in single combat wearing a full set of body armour, I began to wonder when a knight dressed as such would've been a thing of the past on the battlefield. I'm assuming it closely followed gun powder and subsequently muskets and the like becoming wide-spread but I'd imagine it took a lot of nations time to adjust to the warfare altering effects of such an invention, much like the way we're taught the tactics of World War One lagged behind the speedy growth of humans abilities to kill one another.
It is traditionally stated that armor was made obsolete by gunpowder weapons, however more than outright obsolescence, the phasing out of armor is better seen as a slow evolution towards tactics relying on ever less armored soldiers, becase there was a very significant transitional period during which we cannot really state that armor was merely being discontinued, if anything armor was used as much and was just as practical during much of the musket era as it was before.
The fully armored knight usually seen in popular media depictions was actually contemporary with infantry firearms in late medieval and early modern times. They were used as heavy lancers crashing against enemy lines like in traditional medieval tactics. However due to the influence of firearms these tactics changed, this influence goes in two ways, not only in the sense that infantry fire was particularly effective against armored opponents, but that usage of firearms by cavalry itself shaped their tactics. The caracole tactic was developed in the 16th century and used extensively as a way of integrating the firepower of the new weapons for cavalry use. Horsemen would take turns firing their pistols in ranks to disrupt enemy formations and then they would charge. So quickly armored calvary had to contend in a battlefield saturated by fire. As a response armor became even heavier to be able to resist pistol fire and infantry fire at long ranges, and this increase in weight combined with a shift in focus in mobility to carry out more complicated manouvers in battle, moreso than offensive effectiveness of firearms themselves, led to horsemen adopting what is called three quarters armor, dropping leg and lower back armor, as well as using an open helmet as well as characteristically blackened to prevent rust. The heaviest armor set could weight up to 42kg/92lb.
Basically there were two types of cavalry for combat, one focused more on shock tactics and one focused more on firepower. The first includes the traditional lancer and later demilancer in 3/4 armor, as well as the cuirassier armed with pistols and sword. The other type of cavalry was variously called harquebusier, pistolier, carabinier etc. which attests to their main weaponry being firearms, shorter than infatry firearms as well as several pistols, and often using weaker armor than the cuirassiers and dropping arm protection.
As the 16th century gave way to the 17th infantry firepower increased due to the increase of shot to pike ratio as well as the introduction of the stronger musket, this combined with improvements in armor made the caracole tactic less reliable. As the harassing use of firearms decreased, they remained in use in shock tactics, basically being used as glorified melee weapons. Cavalry would approach the enemy and shoot their weapons at the closest range possible, barrel of the pistol literally pressed against enemy breastplate kind of close. During this century armor suffered more cuts, as their increased weight and cost were becoming too costly for the decreasing benefit they were providing, though armored units naturally still performed better than unarmored ones, better protected and thus less likely to turn and flee. Cavalry would wear half armor and the lighter units just a leather buff coat.
In the 18th century shot was all mighty, the introduction of the bayonet led to the demise of the pikeman and the universal adoption of the flintlock meant that infantry firepower was inescapable, By this period only the cuirassiers retained any sort of armor, reserved to the breastplate, as it was still useful against sword and pistol attacks, and armies would discontinue and reissue it throughout the century. Do bear in mind that while heavily armored cavalry dissapeared, heavy cavalry raised for the purposes of delivering frontal charges were still very much a thing, and would remain so until the technological advancements in mid to late 19th century made such a frontal assault mostly suicidal.
Infantry armor shared a similar path to eventual disuse, as armor was worn to protect head and torso during fights against other pikemen and calvary, but as infantry firepower increased it was left behind duriing the mid to late 17th century, as pikemen weren't as heavily armored to being with and were less capable of affording the terribly expensive armor that the often aristocratic horseman could, and with the adoption of the bayonet fusing heavy and light infantry into one infantry armor was rendered moot.
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