  Transition of the ground state of a classical $\Phi^4$ theory in 2+1
dimensions is studied from a metastable state into the stable equilibrium. The
transition occurs in the broken $Z_2$ symmetry phase and is triggered by a
vanishingly small amplitude homogeneous external field $h$. A phenomenological
theory is proposed in form of an effective equation of the quantitatively
accounts for the decay of the false vacuum. The large amplitude transition of
the order parameter between the two minima displays characteristics reflecting
dynamical aspects of the Maxwell construction.
