  Quantum phase transitions induced by an external magnetic field in the
Haldane-gapped spin-1 chains are studied in a fermionic field-theoretic
description of the model. In the case with broken axial symmetry, two
transitions occurs consecutively as increasing the field: $h_c=(m_1 m_2)^{1/2}$
(with $m_1$ and $m_2$ the transverse masses) is the critical Ising point and
$\tilde{h}_c^* (> h_c)$ is the commensurate-incommensurate transition point.
Although the latter is thermodynamically indiscernible, its property is
revealed in the spin correlation functions (both uniform and staggered) which
are calculated upon expressing the correlations of coupled Ising pair fields in
terms of block-Toeplitz determinants.
