Two important observational constraints on substorm onset theories are that (1) substorm onset occurs on field lines of a pre-existing auroral arc, and (2) reduction of the cross-tail current occurs in the near-Earth magnetotail. Recent progress in linking generation of at least some auroral arcs to field line resonances (FLRs) and current disruption to possibly the cross-field current instability (CCI) suggests a synergistic combination of these two theories. We suggest that as the cross-tail current sheet thins to the extent that ions become unmagnetized at the end of substorm growth phase, the equatorial electric field associated with FLRs can amplify significantly the cross-tail current to exceed the instability threshold. This combined theory naturally accounts for the observational constraints of substorm onset starting on a pre-existing auroral arc and linking closely with current reduction in the near-Earth magnetotail. It is compatible with the observation of the most equatorward auroral arc being the most likely one for initial brightening because the cross-tail current is most intense in its near-Earth end and is therefore most likely to exceed the unstable threshold for CCI. It is also consistent with auroral precipitation fading to be a common feature before an auroral breakup. Other predictions yet to be tested are that (1) the onset is more favorable for the phase of the resonance in which the resonance-associated field-aligned current is directed from the ionosphere to the magnetosphere, and (2) the onset is more favorable poleward of the resonance shell or auroral arc. ¿ 1998 American Geophysical Union