  A mechanism for addressing the 'decompactification problem' is proposed,
which consists of balancing the vacuum energy in Scherk-Schwarzed theories
against contributions coming from non- perturbative physics. Universality of
threshold corrections ensures that, in such situations, the stable minimum will
have consistent gauge couplings for any gauge group that shares the same N = 2
beta function for the bulk excitations as the gauge group that takes part in
the minimisation. Scherk- Schwarz compactification from 6D to 4D in heterotic
strings is discussed explicitly, together with two alternative possibilities
for the non-perturbative physics, namely metastable SQCD vacua and a single
gaugino condensate. In the former case, it is shown that modular symmetries
gives various consistency checks, and allow one to follow soft-terms, playing a
similar role to R-symmetry in global SQCD. The latter case is particularly
attractive when there is nett Bose-Fermi degeneracy in the massless sector. In
such cases, because the original Casimir energy is generated entirely by
excited and/or non-physical string modes, it is completely immune to the
non-perturbative IR physics. The large separation between UV and IR
contributions to the potential greatly simplifies the analysis of
stabilisation, and is a general possibility that has not been considered
before.
