  We establish a variational framework for nonlinear instabilities in a setting
of the ideal magnetohydrodynamic (MHD) equations. We apply a variational method
to various kind of smooth steady states which are shown to be nonlinearly
unstable for both incompressible and compressible ideal MHD equations.
Destabilizing effect of compressibility is justified as well as stabilizing
effect of magnetic field lines arising in MHD dynamics, which distinguishes
from the Rayleigh-Taylor instability in the absence of magnetic field lines.
