  We use the chiral Lagrangian to investigate the global properties of the
$N$-flavor QCD vacuum as a function of the $\theta$ parameter. In the case of
exact quark degeneracy we find evidence for first order phase transitions at
$\theta = \pi \cdot ({\rm odd~ integer})$. The first order transitions are
smoothed by quark mass splittings, although interesting effects remain at
realistic quark masses. We emphasize the role of the $\eta'$ condensate in our
analysis. Finally, we discuss the implications of our results on the internal
hadronic structure of axion domain walls and axion cosmology.
