  On the basis of the three fundamental principles of (i) Poincar\'{e} symmetry
of space time, (ii) electromagnetic gauge symmetry, and (iii) unitarity, we
construct an universal Lagrangian for the electromagnetic interactions of
elementary vector particles, i.e., massive spin-1 particles transforming in the
/1/2,1/2) representation space of the Homogeneous Lorentz Group (HLG). We make
the point that the first two symmetries alone do not fix the electromagnetic
couplings uniquely but solely prescribe a general Lagrangian depending on two
free parameters, here denoted by \xi and g. The first one defines the
electric-dipole and the magnetic-quadrupole moments of the vector particle,
while the second determines its magnetic-dipole and electric-quadrupole
moments. In order to fix the parameters one needs an additional physical input
suited for the implementation of the third principle. As such, one chooses
Compton scattering off a vector target and requires the cross section to
respect the unitarity bounds in the high energy limit. In result, we obtain the
universal g=2, and \xi=0 values which completely characterize the
electromagnetic couplings of the considered elementary vector field at tree
level. The nature of this vector particle, Abelian versus non-Abelian, does not
affect this structure. Merely, a partition of the g=2 value into non-Abelian,
g_{na}, and Abelian, g_{a}=2-g_{na}, contributions occurs for non-Abelian
fields with the size of g_{na} being determined by the specific non-Abelian
group appearing in the theory of interest, be it the Standard Model or any
other theory.
