  We investigate cooperative fluorescence in a dilute cloud of strongly driven
two-level emitters. Starting from the Heisenberg equations of motion, we
compute the first-order scattering corrections to the saturation of the
excited-state population and to the resonance-fluorescence spectrum, which both
require going beyond the state-of-the-art linear-optics approach to describe
collective phenomena. A dipole blockade is observed due to long range
dipole-dipole coupling that vanishes at stronger driving fields. Furthermore,
we compute the inelastic component of the light scattered by a cloud of many
atoms and find that the Mollow triplet is affected by cooperativity. In a lobe
around the forward direction, the inelastic Mollow triplet develops a spectral
asymmetry, observable under experimental conditions.
