  We analyze the dependence of the QCD transition temperature on the quark (or
pion) mass. We find that a linear sigma model, which links the transition to
chiral symmetry restoration, predicts a much stronger dependence of T_c on m_pi
than seen in present lattice data for m_pi >~ 0.4 GeV. On the other hand,
working down from m_pi=infinity, an effective Lagrangian for the Polyakov loop
requires only small explicit symmetry breaking, b_1 ~ exp(-m_pi), to describe
T_c(m_pi) in the above mass range. Physically, this is a consequence of the
flat potential (large correlation length) for the Polyakov loop in the
three-color pure gauge theory at T_c. We quantitatively estimate the end point
of the line of first order deconfining phase transitions: m_pi = 1.8 GeV and Tc
= 240 MeV for three flavors and three colors.
