Short answers, pulled from the story.
Military history is the study of armed conflict across human history and its impact on societies, cultures, and economies. Its essential subjects include the causes of war, military doctrine, logistics, leadership, technology, strategy, and tactics, and how these changed over time.
William H. McNeill described military history as flourishing in an intellectual ghetto despite its popularity with the public. Of 144 books published between 1968 and 1978, he found they split into popular works by journalists and men of letters and professional work produced within the military establishment, with university study seriously underdeveloped.
Past is Prologue is the motto the Combat Studies Institute uses when certifying military history instructors. The institute deemphasizes rote detail memorization and focuses on themes and context in relation to current and future conflict, so commanders can perceive historical parallels during a battle.
The Society for Military History is the scholarly organization for academic historians concerned with military topics, and since 1937 it has published The Journal of Military History. The journal runs four issues a year, and the Society has 2,300 members, holds an annual convention, and awards prizes for the best scholarship.
The Battle of Megiddo, recorded by the Egyptian military scribe Tjaneni in the 15th century BC, is accepted as the first battle preserved in relatively reliable detail. Earlier documentation of conflict reaches back to clashes around 3150 BC in Egypt and around 2700 BC between Sumer and Elam.
Oda Nobunaga successfully implemented the volley or countermarch technique in 1575, twenty years before such a technique appears in Europe, where it first showed up in the mid-1590s with Lord William Louis of Nassau. Korea may have used a volley principle even earlier, by 1447 under King Sejong the Great.