Short answers, pulled from the story.
Military tactics rests on four functions that are closely related: kinetic force or firepower, mobility, protection or security, and shock action. Throughout history, whichever function has been dominant in a given period has usually elevated a particular fighting arm, such as infantry, cavalry, artillery, or tanks.
Infantry-delivered missile firepower was relatively weak until the development of disciplined volley fire. The introduction of the rifled musket, used in the Crimean War and the American Civil War, then dramatically improved range and accuracy. Gunpowder accelerated this shift across both European and East Asian warfare, with the Battle of Nagashino in 1575 often cited as a landmark moment for massed arquebusier fire.
Before World War I, armies were limited to the walking pace of a soldier on foot. The tank, introduced in the latter stages of World War I, was the first major break from that constraint. Full tactical mobility only arrived in World War II with armoured and motorised formations, though large portions of those armies still depended on horse-drawn transport.
Shock action is as much a psychological weapon as a physical one. Charging infantry, war elephants, cavalry, and armoured vehicles all create momentum and psychological disruption that can break an enemy's cohesion before significant casualties are inflicted. At the Battle of Agincourt in 1415, longbow fire panicked French cavalry horses, demonstrating that shock can work defensively as well as offensively.
Gray-zone tactics cover actions that sit in the ambiguous space between peace and war. They include strong-arm diplomacy, economic coercion, media manipulation, cyberattacks, and the use of paramilitaries and proxy forces. The name reflects the deliberate blurring of the line between offensive and defensive action.
By the 18th and 19th centuries the weight and bulk of armour had become too costly in terms of mobility and endurance, so it was largely discarded. World War I artillery firepower forced the reintroduction of helmets. Armoured fighting vehicles proliferated in World War II, and infantry body armour returned after that war, particularly in Western armies.