  We study gluon production in high energy proton-nucleus collisions in the
semi-classical framework of the Color Glass Condensate. We develop a general
formalism to compute gluon fields in covariant gauge to lowest order in the
classical field of the proton and to all orders in the classical field of the
nucleus. The use of the covariant gauge makes the diagrammatic interpretation
of the solution more transparnt. k_t-factorization holds to this order for
gluon production -- Our results for the gluon distribution are equivalent to
the prior diagrammatic analysis of Kovchegov and Mueller. We also show that
these results are equivalent to the computation of gluon production by Dumitru
and McLerran in the Fock-Schwinger gauge. We demonstrate how the Cronin effect
arises in this approach, and examine its behavior in the two extreme limits of
a) no small-x quantum evolution, and b) fully saturated quantum evolution. In
both cases, the formalism reduces to Glauber's formalism of multiple
scatterings. We comment on the possible implications of this study for the
interpretation of the recent results on Deuteron-Gold collisions at the
Relativistic Heavy Ion Collider (RHIC).
