Theta, Time Reversal, and Temperature
$SU(N)$ gauge theory is time reversal invariant at $θ=0$ and $θ=π$. We show that at $θ=π$ there is a discrete 't Hooft anomaly involving time reversal and the center symmetry. This anomaly leads to constraints on the vacua of the theory. It follows that at $θ=π$ the vacuum cannot be a trivial non-degenerate gapped state. (By contrast, the vacuum at $θ=0$ is gapped, non-degenerate, and trivial.) Due to the anomaly, the theory admits nontrivial domain walls supporting lower-dimensional theories. Depending on the nature of the vacuum at $θ=π$, several phase diagrams are possible. Assuming area law for space-like loops, one arrives at an inequality involving the temperatures at which CP and the center symmetry are restored. We also analyze alternative scenarios for $SU(2)$ gauge theory. The underlying symmetry at $θ=π$ is the dihedral group of 8 elements. If deconfined loops are allowed, one can have two $O(2)$-symmetric fixed points. It may also be that the four-dimensional theory around $θ=π$ is gapless, e.g. a Coulomb phase could match the underlying anomalies.