  We completely compute the local BRST cohomology $H(s|d)$ of the combined
Yang-Mills-2-form system coupled through the Yang-Mills Chern-Simons term
("Chapline-Manton model"). We consider the case of a simple gauge group and
explicitely include in the analysis the sources for the BRST variations of the
fields ("antifields"). We show that there is an antifield independent
representative in each cohomological class of $H(s|d)$ at ghost number 0 or 1.
Accordingly, any counterterm may be assumed to preserve the gauge symmetries.
Similarly, there is no new candidate anomaly beside those already considered in
the literature, even when one takes the antifields into account. We then
characterize explicitly all the non-trivial solutions of the Wess-Zumino
consistency conditions. In particular, we provide a cohomological
interpretation of the Green-Schwarz anomaly cancellation mechanism.
