Electromagnetic pulses (EMP) driven by lightning can cause breakdown of the neutral atmosphere in the lower D-region. Using a computer simulation model, we study the dependence of the breakdown on the pulse strength, the orientation of the lightning discharge, the ambient plasma density, the ionization model, and the neutral density. For a discharge along a straight line the EMP is strongest in the plane perpendicular to the current so that for a given current, horizontal discharges will radiate the D-region more strongly than a vertical discharge. For horizontal currents, breakdown occurs for E100>20 V/m (I>55 kA) in a low-density, nighttime ionosphere, where E100 is the amplitude of the pulse normalized to 100 km from the discharge and I is the discharge current. Vertical strokes require E100>50 V/m (I>140 kA). Discharges with higher currents and fields form ionization patches which are larger in volume, larger in degree of ionization, and lower in altitude. The ionization is most sensitive to the pulse strength, pulse orientation, ambient plasma density, and neutral gas density at breakdown threshold. Higher ambient plasma densities reduce the ionization, but for large EMP, breakdown can occur even with high daytime densities. The breakdown increases the plasma density which acts to limit the EMP and ionization. This feedback reduces the sensitivity of the breakdown to the ionization model. Neutral density variations, such as caused by atmospheric gravity waves, can cause spatial variations in the ionization density.