We consider QCD at theta similar to pi with two, one and zero light flavors
N-f, using the Di Vecchia-Veneziano-Witten effective Lagrangian. For N-f=2
, we show that CP is spontaneously broken at theta=pi for finite quark mass
splittings, z=m(d)/m(u)not equal 1. In the z - theta plane, there is a lin
e of first order transitions at theta=pi with two critical end points, z(1)
*<z<z(2)*. We compute the tension of the domain walls that relate the two C
P violating vacua. For m(u) = m(d), the tension of the family of equivalent
domain walls agrees with the expression derived by Smilga from chiral pert
urbation theory at next-to-leading order. For z(1)*<z<z(2)*, z not equal 1,
there is only one domain wall and a wall-some sphaleron at theta=pi. At th
e critical points, z=z(1,2)* the domain wall fades away, CP is restored and
the transition becomes of second order. For N-f=1, CP is spontaneously bro
ken only if the number of colors N-c is large and/or if the quark is suffic
iently heavy. Taking the heavy quark limit (similar to N-f =0) provides a s
imple derivation of the multibranch theta dependence of the vacuum energy o
f large N-c pure Yang-Mills theory. In the large N-c limit, there are many
quasistable vacua with a decay rate Gamma similar to exp(-N-c(4)).