  We study, by means of MHD simulations, the onset and evolution of fast
reconnection via the "ideal" tearing mode within a collapsing current sheet at
high Lundquist numbers ($S\gg10^4$). We first confirm that as the collapse
proceeds, fast reconnection is triggered well before a Sweet-Parker type
configuration can form: during the linear stage plasmoids rapidly grow in a few
Alfv\'en times when the predicted "ideal" tearing threshold $S^{-1/3}$ is
approached from above; after the linear phase of the initial instability,
X-points collapse and reform nonlinearly. We show that these give rise to a
hierarchy of tearing events repeating faster and faster on current sheets at
ever smaller scales, corresponding to the triggering of "ideal" tearing at the
renormalized Lundquist number. In resistive MHD this process should end with
the formation of sub-critical ($S \leq10^4$) Sweet Parker sheets at microscopic
scales. We present a simple model describing the nonlinear recursive evolution
which explains the timescale of the disruption of the initial sheet.
