Which of these is the leading part of an advancing military formation?
The vanguard (also called the advance guard) is the leading part of an advancing military formation. It has a number of functions, including seeking out the enemy and securing ground in advance of the main force.
The vanguard derives from the traditional division of a medieval army into three battles or wards; the Van, the Main (or Middle), and the Rear. The term Vanguard originates with the medieval French avant-garde, i.e. "the advance guard". The vanguard would lead the line of march and would deploy first on the field of battle, either in front of the other wards or to the right if they stood in line.