  Global geometry of $K3$-fibration Calabi-Yau threefolds, with Hodge number
$h_{2,1}=r+1$, is used to define $N=4$ softly broken $SU(r+1)$ gauge theories,
with the bare coupling constant given by the dual heterotic dilaton, and the
mass of the adjoint hypermultiplet given by the heterotic string tension. The
$U(r+1)$ Donagi-Witten integrable model is also derived from the $K3$-fibration
structure, with the extra $U(1)$ associated to the heterotic dilaton. The case
of $SU(2)$ gauge group is analyzed in detail. String physics beyond the
heterotic point particle limit is partially described by the $N=4$ softly
broken theory.
