  We compute the 1-loop (alpha_s^2) correction to hard spectator scattering in
non-leptonic B decay tree amplitudes. This forms part of the NNLO contribution
to the QCD factorization formula for hadronic B decays, and introduces a new
rescattering phase that corrects the leading-order result for direct CP
asymmetries. Among the technical issues, we discuss the cancellation of
infrared divergences, and the treatment of evanescent four-quark operators. The
infrared finiteness of our result establishes factorization of spectator
scattering at the 1-loop order. Depending on the values of hadronic input
parameters, the new 1-loop correction may have a significant impact on
tree-dominated decays such as B -> pi pi.
