  We explicitly describe, in the language of four-dimensional N=1
supersymmetric field theory, what happens when the moduli of a heterotic
Calabi-Yau compactification change so as to make the internal non-Abelian gauge
fields non-supersymmetric. At the edge of the region in Kahler moduli space
where supersymmetry can be preserved, an additional anomalous U(1) gauge
symmetry appears in the four-dimensional theory. The D-term contribution to the
scalar potential associated to this U(1) attempts to force the system back into
a supersymmetric configuration and provides a consistent low-energy description
of gauge bundle stability.
