  We study the effective low energy supergravity of the strongly coupled
heterotic string compactified on a Calabi-Yau 3-fold with generic E8 x E8 gauge
bundle. We focus on the effective potential for the chiral scalars. The
effective superpotential can be studied using the dual 11-dimensional M-theory
background involving insertions of M5 branes along an interval. In such
backgrounds, in some regions of moduli space, the leading nonperturbative
contributions are due to open membrane instantons. These instantons lead to
both attractive and repulsive forces between the 5-branes and the orientifold
``M9-branes,'' depending on the region of moduli space. The resulting dynamics
on moduli space include a strong coupling dual to the Dine-Seiberg instability,
in which the interval grows. We discuss conditions under which the 5-branes are
attracted to the wall and comment on the relevance of these results to the
study of chirality-changing phase transitions in heterotic M-theory.
