  Motivated by the instability of the Savvidy-Nielsen-Olesen (SNO) vacuum we
make a systematic search for a stable magnetic background in pure SU(2) QCD. It
is shown that Wu-Yang monopole-antimonopole pair is unstable under vacuum
fluctuations. However, it is shown that a pair of axially symmetric
monopole-antimonopole string configuration is stable, provided the distance
between the two strings is small enough (less than a critical value). The
existence of a stable monopole-antimonopole string background strongly supports
that a magnetic condensation of monopole-antimonopole pairs can indeed generate
a dynamical symmetry breaking, and thus a desired magnetic confinement of
color, in QCD.
