  The theta dependent of pure gauge theories in four dimensions can be studied
using a duality of large N gauge theories with string theory on a certain
spacetime. Via this duality, one can argue that for every theta, there are
infinitely many vacua that are stable in the large N limit. The true vacuum,
found by minimizing the energy in this family, is a smooth function of theta
except at theta equal to pi, where it jumps. This jump is associated with
spontaneous breaking of CP symmetry. Domain walls separating adjacent vacua are
described in terms of wrapped sixbranes.
