The excitation of plasma waves by two laser beams, whose frequency difference is approximately the plasma frequency, is analyzed. Our nonlinear analysis is fully relativistic and includes mismatching of the laser beat frequency to the plasma frequency, time dependent laser amplitudes, and an applied transverse magnetic field (surfatron). For a given beat frequency, laser power, and plasma density, we find the peak accelerating electric field and its phase velocity. The transverse magnetic field is found to increase the effective plasma frequency, but has little effect on the plasma dynamics.