  We present here a nonlinear sigma-model (NL$\sigma$M) study of a spin-1
antiferromagnetic Heisenberg chain in an external commensurate staggered
magnetic field. We find, already at the mean-field level, excellent agreement
with recent and very accurate Density Matrix Renormalization Group (DMRG)
studies, and that up to the highest values of the field for which a comparison
is possible, for the staggered magnetization and the transverse spin gap.
Qualitative but not quantitative agreement is found between the NL$\sigma$M
predictions for the longitudinal spin gap and the DMRG results. The origin of
the discrepancies is traced and discussed. Our results allow for extensions to
higher-spin chains that have not yet been studied numerically, and the
predictions for a spin-2 chain are presented and discussed. Comparison is also
made with previous theoretical approaches that led instead to predictions in
disagreement with the DMRG results.
