  We study the spin-boson model with a sub-Ohmic bath using a variational
method. The transition from coherent dynamics to incoherent tunneling is found
to be abrupt as a function of the coupling strength $\alpha$ and to exist for
any power $0 < s< 1$, where the bath coupling is described by $J(\omega) \sim
\alpha \omega^{s}$. We find non-monotonic temperature dependence of the
two-level gap $\tilde{K}$ and a re-entrance regime close to the transition due
to non-adiabatic low-frequency bath modes. Differences between thermodynamic
and dynamic conditions for the transition as well as the limitations of the
simplified bath description are discussed.
