The nonlinear transient evolution of a suddenly applied monochromatic wave in a homogeneous plasma is considered, with particular emphasis on the magnetized case, where ordinary and extraordinary electromagnetic waves travelling normal to B are treated. Two important features are described. First, the penetration of the wave front is shown to be accompanied by ‘radiation’ of low-frequency waves, of which ion cyclotron and lower-hybrid waves are considered here. Next, the presence of a nonlinear, second-harmonic precursor is predicted, containing two natural modes of oscillation, one with exactly twice the frequency of the fundamental, and the other a slightly frequency-shifted contribution. The analysis is readily generalized to waves other than those considered here.