  We derive the ten-dimensional effective action of the strongly coupled
heterotic string as the low energy limit of M-theory on S^1/Z_2. In contrast to
a conventional dimensional reduction, it is necessary to integrate out
nontrivial heavy modes which arise from the sources located on the orbifold
fixed hyperplanes. This procedure, characteristic of theories with dynamical
boundaries, is illustrated by a simple example. Using this method, we determine
a complete set of R^4, F^2R^2, and F^4 terms and the corresponding Chern-Simons
and Green-Schwarz terms in ten dimensions. As required by anomaly cancelation
and supersymmetry, these terms are found to exactly coincide with their weakly
coupled one-loop counterparts.
