  We investigate the effect of a homogeneous magnetic field on the thermal
deconfinement transition of QCD in the large $N_c$ limit. First we discuss how
the critical temperature decreases due to the inclusion of $N_f \ll N_c$
flavors of massless quarks. Then we study the equivalent correction in the
presence of an external Abelian magnetic field. To leading order in
$N_{f}/N_{c}$, the fact that the deconfinement critical temperature decreases
with the magnetic field depends solely on quarks behaving paramagnetically.
Finally, we discuss the effects from a finite quark mass and its competition
with magnetic effects.
