  The pattern of chiral symmetry breaking is exploited to compute vector and
axial-vector pion matrix elements in a uniform magnetic field. Our results are
model independent, and thereby constitute low-energy theorems that must be
obeyed by QCD in external magnetic fields. Chiral perturbation theory and
lattice QCD results are compared, for which there is some tension. As an
application, the matrix elements are utilized to compute pion decay rates in a
magnetic field.
