We present in this paper an exact study concerning a first order transition
induced by an inhomogeneous boundary magnetic field in the 2D Ising model. From
a previous analysis of the interfacial free energy in the discrete case (J.
Phys. A, 2849, 2005), we identify, using an asymptotic expansion in the
thermodynamic limit, the transition line that separates the regime where the
interface is localised near the boundary from the regime where it is
propagating inside the bulk. In particular, the critical line has a strong
dependence on the aspect ratio of the lattice.