Two-dimensional simulations of the magnetotail in the high-Lundquist-number regime indicate the slow growth of thin current sheets and an impulsive intensification of the cross-tail current density at near-Earth distances during a short interval just before the onset of the expansion phase, consistent with multisatellite observations. Such a two-dimensional magnetotail, symmetric along y and containing a thin current sheet, is found to be unstable to a symmetry-breaking, ideal compressible ballooning instability with high wave number along y. The linear instability is demonstrated by numerical solutions of the ideal ballooning eigenmode equation for a sequence of two-dimensional thin current sheet configurations in the impulsive growth phase. Line-tied boundary conditions at the ionosphere are imposed, and shown to play a crucial role in the stability analysis. It is suggested that the ideal ballooning instability, which has strong spatial variation along y, provides a possible mechanism for disrupting the cross-tail current at onset. ¿ 1998 American Geophysical Union