  Electromagnetic force and torque are typically derived from a stress tensor
in conjunction with Maxwell's equations of classical electrodynamics. In some
instances, the Principle of Least Action (built around a Lagrangian) can be
used to arrive at the same mathematical expressions of force and torque as
those derived from a stress tensor. This paper describes some of the underlying
arguments for the existence of a Lagrangian in the case of certain simple
physical systems. While some formulations of electromagnetic force and torque
admit a Lagrangian, there are other formulations for which a Lagrangian may not
exist.
