  While it is clear that in some kinematic regime QCD can be described by an
effective (as opposed to fundamental) string theory, it is not at all clear how
this string theory should be. The `natural' candidate, the bosonic string,
leads to amplitudes with the usual problems related to the existence of the
tachyon, absence of the adequate Adler zero, and massless vector particles, not
to mention the conformal anomaly. The supersymmetric version does not really
solve most of these problems. For a long time it has been believed that the
solution of at least some of these difficulties is associated to a proper
identification of the vacuum, but this program has remained elusive. We show in
this work how the first three problems can be avoided, by using a sigma model
approach where excitations above the correct (chirally non-invariant) QCD
vacuum are identified. At the leading order in a derivative expansion we
recover the non-linear sigma model of pion interactions. At the next-to-leading
order the O(p^4) Lagrangian of Gasser and Leutwyler is obtained, with values
for the coefficients that match the observed values. We also discuss some
issues related to the conformal anomaly.
