Lower Critical Dimension of the Random-Field Ising Model

A new argument is given for a lower critical dimension $d_l=2\mathrm{}$ for the Ising model in a random magnetic field. It forms the basis for a proof that the three-dimensional model exhibits long-range order at zero temperature and small disorder. This settles the controversy between the values $d_l=2\mathrm{}$ and $d_l=3\mathrm{}$.