Short answers, pulled from the story.
The four core functions of military tactics are kinetic force, movement, protection, and the psychological shock of the charge. These elements have remained constant throughout history even as technology changed the weapons used to deliver them.
Infantry-delivered missile firepower began to rise significantly in the mid 19th century. The introduction of the rifled musket during the Crimean War and the American Civil War marked a turning point that offered flatter trajectories and improved accuracy at greater ranges.
The shock cycle dominated military tactics between 650 and 1450 according to J. F. C. Fuller. This period was followed by a shock and projectile cycle from 1450 to 1850 and a projectile cycle from 1850 to the present.
The Battle of Plataea in 479 BC ended the first unarmored period in Tom Wintringham's chronological periods. The first armored period concluded with the Battle of Adrianople in 378.
Personal armor was largely discarded by the 18th and 19th centuries because its weight and bulk reduced soldier mobility and endurance. It was re-introduced during World War I in response to the firepower of artillery.
The gray zone in modern military tactics is defined by the ambiguity between defense versus offense and peace-keeping versus war effort. These tactics include strong-arm diplomacy, economic coercion, media manipulation, and cyberattacks.