  A system of coupled kinetic transport equations for the Wigner distributions
of a free variable mass Klein-Gordon field is derived. This set of equations is
formally equivalent to the full wave equation for electromagnetic waves in
nonlinear dispersive media, thus allowing for the description of broadband
radiation-matter interactions and the associated instabilities. The standard
results for the classical wave action are recovered in the short wavelength
limit of the generalized Wigner-Moyal formalism for the wave equation.
