The Sergeant started out as a servant, serviens in Latin, to a knight in medieval times. He became a fighting man probably for self preservation because combat in those days often amounted to cutting down everybody in reach, regardless of whether they were armed. He became an experienced warrior who might ride a horse but was not wealthy enough to afford all the equipment and retainers to qualify as a knight. As an experienced soldier he might be called upon to take charge of a group of serfs or other common people forced to serve in an army of feudal levies. The Sergeant would conduct what training he could to teach his charges to fight, lead them into battle and, most important, keep them from running away during a battle. Sergeant was not a rank but an occupation. He might lead others he might fight alone or as a member of a group of sergeants, or he might serve the lord of his village as a policeman or guard. The modern title "sergeant-at-arms" used by many clubs recalls armed Sergeants who kept order at meetings.
The English borrowed the word "sergeant" from the French in about the Thirteenth Century. They spelled it several different ways and pronounced it both as SARgent and SERgeant. The latter was closer to the French pronunciation. The SARgeant pronunciation became the most popular, however, so that when the Nineteenth Century dictionary writers agreed that the word should be spelled "sergeant" they could not change the popular pronunciation. Thus, we say SARgeant while the French and others say SERgeant.
Sergeant became a regular position and then a rank as army organizations evolved. It has been a key rank in British and European armies for several hundred years. When our Army and Marine Corps started in 1775 it was natural that both include Sergeants. The rank's many duties and levels of responsibility have lead to several grades of Sergeant. The Air Force used to have six grades of sergeant, while the Army and the Marines only had five. The sixth grade was a "Buck" Sergeant (E-4). Since the dual (E-4) rank of Senior Airman and Sergeant proved confusing to the other branches of service and did not include more pay and only rarely more responsibility, the Air Force promoted its last Senior Airman to "Buck" Sergeant in May 1990 and phased the rank out over the next six years. At present the Air Force, Army and Marines all have five grades of Sergeant ranging from (E-5) to (E-9).