The effective‐field concept is used to derive a breakdown criterion for a model gas subjected to strong laser and microwave fields. This classical model includes diffusion, collisions, and the finite pulse length. The pressure dependence of this criterion is explored, and it is found that, under the proper conditions, it is possible to produce one or two narrow pressure regions in which breakdown will occur, or two broad breakdown regions separated by an extremely narrow nonbreakdown region. The criterion is extended to account for time‐dependent field intensities and frequencies.