  The gravitational waves from the first binary neutron star merger, GW170817,
were accompanied by a multi-wavelength electromagnetic counterpart, from
$\gamma$-rays to radio. The accompanying gamma-rays, seems at first to confirm
the association of mergers with short gamma-ray bursts (sGRBs). The common
interpretation was that we see an emission from an sGRB jet seen off-axis.
However, a closer examination of the sub-luminous $\gamma$-rays and the
peculiar radio afterglow were inconsistent with this simple interpretation.
Here we present results of 3D and 2D numerical simulations that follow the
hydrodynamics and emission of the outflow from a neutron star merger form its
ejection and up to its deceleration by the circum-merger medium. Our results
show that the entire set of $\gamma$-rays, X-rays and radio observations can be
explained by the emission from a mildly relativistic cocoon material (Lorentz
factor $\sim$2-5) that was formed while a jet propagated through the material
ejected during the merger. The $\gamma$-rays are generated when the cocoon
breaks out from the engulfing ejecta while the afterglow is produced by
interaction of the cocoon matter with the interstellar medium. The strong early
uv/optical signal may be a Lorentz boosted macronova/kilonova. The fate of the
jet itself is currently unknown, but our full-EM models define a path to
resolving between successful and choked jet scenarios, outputting coupled
predictions for the image size, morphology, observed time-dependent
polarization and light curve behavior from radio to X-ray. The predictive power
of these models will prove key in interpreting the on-going multi-faceted
observations of this unprecedented event.
