  The similarities of hadrons and atoms motivate a study of the principles of
QED bound states and of their applicability to QCD. The power series in
$\alpha$ and $\log\alpha$ of the binding energy is reflected in the Fock
expansion of the bound state in temporal gauge ($A^0=0$). Gauss' constraint on
physical states fixes the gauge for time independent transformations and
determines the instantaneous interaction within each Fock state.
  Positronium atoms generate a classical (dipole) electric field, whereas there
can be no color octet gluon field for color singlet hadrons. Hence the gluon
field generated by each color component of a hadron need not vanish at spatial
infinity. Gauss' constraint has a homogeneous solution with a single parameter
$\Lambda$ that is compatible with Poincar\'e invariance. The corresponding
potential is linear for $q\bar q$ and $gg$ Fock states, and confining also for
other states ($q\bar qg,\,qqq$).
  This approach is consistent with the quarkonium phenomenology based on the
Cornell potential at lowest order. The relativistic meson and glueball
eigenstates of the QCD Hamiltonian with the $O(\alpha_s^0)$ linear potential
are determined. The states lie on linear Regge trajectories and their
daughters. There are also massless bound states which allow to include a
$J^{PC}=0^{++}$ condensate in the perturbative vacuum, thus breaking chiral
symmetry spontaneously.
