Which command is used to indicate the onset of marching in military exercises?

The command used to indicate the onset of marching in military exercises is "March." This command signifies to the troops that they should start moving in an organized manner, typically in unison. It is a direct and universally understood instruction within military training and operations, allowing for a synchronized movement that is crucial during formations and drills.

Other commands such as "Advance" and "Forward" may imply a movement towards a target or movement in a specific direction, but they do not specifically invoke the structured initiation of marching as "March" does. "Begin" is also a more general term that lacks the established connotation of coordinated movement that "March" embodies. Thus, "March" is the most appropriate term to initiate a marching sequence in military contexts.