Short answers, pulled from the story.
The Battle of Kadesh in 1274 BC from the reign of Ramses II stands as one of the earliest recorded battles with detailed historical accounts. This conflict between Egypt and the Hittite Empire features in bas-relief monuments.
The United States suspended conscription in 1973 after which the military disproportionately attracted African American men, men from lower-status socioeconomic backgrounds, and men whose high school grades tended to be low. A study released in 2020 suggests that modern U.S. Armed Forces personnel are at parity or slightly higher than the civilian population regarding parental income and cognitive abilities.
One of the oldest military publications is The Art of War by the Chinese philosopher Sun Tzu written in the 6th century BCE. This thirteen-chapter book intended as military instruction has had huge influence on Asian military doctrine and later European planning.
Perhaps the most infamous of all military technologies was creation of nuclear weapons though exact effects were unknown until early 1950s. Gunpowder had been invented in medieval China and increasingly used by the military in combat prior to this development.
An early exponent was Hugo Grotius whose On the Law of War and Peace published in 1625 had major impact on humanitarian approach to warfare development. His theme echoed by Gustavus Adolphus during subsequent conflicts.