  We study theoretically orbital effects of a parallel magnetic field applied
to a disordered superconducting film. We find that the field reduces the phase
stiffness and leads to strong quantum phase fluctuations driving the system
into an insulating behavior. This microscopic model shows that the critical
field decreases with the sheet resistance, in agreement with recent
experimental results. The predictions of this model can be used to discriminate
spin and orbital effects. We find that experiments conducted by A. Johansson
\textit{et al.} are more consistent with the orbital mechanism.
