  The stability of the ideal magnetohydrodynamic (MHD) interchange mode at
marginal conditions is studied. A sufficiently strong constant magnetic field
component transverse to the direction of mode symmetry provides the marginality
conditions. A systematic perturbation analysis in the smallness parameter,
$|b_2/B_c|^{1/2}$, is carried out, where $B_c$ is the critical transverse
magnetic field for the zero-frequency ideal mode, and $b_2$ is the deviation
from $B_c$. The calculation is carried out to third order including nonlinear
terms. It is shown that the system is nonlinearly unstable in the short
wavelength limit, i.e., a large enough perturbation results in instability even
if $b_2/B_c>0$ (linearly stable). The normalized amplitude for instability is
shown to scale as $|b_2/B_c|^{1/2}$. A nonlinear, compressible, MHD simulation
is done to check the analytic result. Good agreement is found, including the
critical amplitude scaling.
