  The QCD partition function for the Wilson Dirac operator, $D_W$, at nonzero
lattice spacing $a$ can be expressed in terms of a chiral Lagrangian as a
systematic expansion in the quark mass, the momentum and $a^2$. Starting from
this chiral Lagrangian we obtain an analytical expression for the spectral
density of $\gamma_5 (D_W+m)$ in the microscopic domain. It is shown that the
$\gamma_5$-Hermiticity of the Dirac operator necessarily leads to a coefficient
of the $a^2$ term that is consistent with the existence of an Aoki phase. The
transition to the Aoki phase is explained, and the interplay of the index of
$D_W$ and nonzero $a$ is discussed. We formulate a random matrix theory for the
Wilson Dirac operator with index $\nu$ (which, in the continuum limit, becomes
equal to the topological charge of gauge field configurations). It is shown by
an explicit calculation that this random matrix theory reproduces the
$a^2$-dependence of the chiral Lagrangian in the microscopic domain, and that
the sign of the $a^2$-term is directly related to the $\gamma_5$-Hermiticity of
$D_W$.
