A growing list of observations and arguments has located the source region of magnetospheric substorms at the near-Earth plasma sheet within 10 RE. A popular descriptive paradigm conceptualizes the substorm initiation as a sudden disruption of a severely stretched magnetic field in the active region. The physical mechanism(s) responsible for the disruption have evoked intense theoretical and computational interests. In this paper we emphasize the macroscopic aspect of substorm onsets and propose to tackle the problem as a two-fluid instability of a plasma significantly polarized by an intense current. It is found that, for a center-maximized current distribution, a drift-wave instability with westward phase speed can grow on a timescale -10 s when the current sheet thins to 1500--3000 km. The phase speed, when mapped to the ionosphere, amounts to 3--5 km/s and is directed westward. The longitudinal size (wavelength) of the fastest growing mode, when similarly mapped, is about 250--500 km. These numbers agree with the morphology of current disruptions and westward traveling surges.¿ 1997 American Geophysical Union