Oscillatory dispersive waves propagating in a slowly varying medium are analyzed for Klein‐Gordon equations with perturbations. The method of multiple scales is extended to include two fast scales, the usual traveling‐wave phase and time, in order to allow initial conditions not usually permitted. An exact wave‐action equation is introduced if the traveling wave is stable, involving averages over the periodic wave as well as time. This is equivalent to an extended averaged Lagrangian principle. The equation for the slow modulations of the phase shift of the traveling wave is derived from the higher order terms in the exact action equation and is shown to be the same as in earlier more restrictive studies.