Turbulence within the plasma sheet of the magnetotail is shown to destabilize the tearing mode even for cases where the equilibrium magnetic field has a normal component within the sheet. In the absence of turbulence, a finite normal magnetic field suppresses the electron wave-particle resonance and stabilizes the tearing mode. Turbulence leads to spatial diffusion of the electron orbits, thus allowing for resonance between the electrons and the tearing perturbation. This provides the dissipation necessary for instability and hence, magnetic field line reconnection. Furthermore, by accounting for the finite extent of the tearing perturbation in the cross-tail direction, the existence of a threshold for instability is derived. This threshold predicts instability for sufficiently small values of the normal magnetic field. Hence, substorm onset proceeds when the turbulent plasma sheet becomes sufficiently elongated. ¿American Geophysical Union 1987