  Casimir interactions are not pair-wise additive. This property leads to
collective effects that we study for a pair of objects near a conducting wall.
We employ a scattering approach to compute the interaction in terms of
fluctuating multipoles. The wall can lead to a non-monotonic force between the
objects. For two atoms with anisotropic electric and magnetic dipole
polarizabilities we demonstrate that this non-monotonic effect results from a
competition between two- and three body interactions. By including higher order
multipoles we obtain the force between two macroscopic metallic spheres for a
wide range of sphere separations and distances to the wall.
