  The Monoceros ring is a collection of stars in nearly-circular orbits at
roughly 18 kpc from the Galactic center. It may have originated (i) as the
response of the disc to perturbations excited by satellite companions or (ii)
from the tidal debris of a disrupted dwarf galaxy. The metallicity of Monoceros
stars differs from that of disc stars at comparable Galactocentric distances,
an observation that disfavours the first scenario. On the other hand, circular
orbits are difficult to accommodate in the tidal-disruption scenario, since it
requires a satellite which at the time of disruption was itself in a nearly
circular orbit. Such satellite could not have formed at the location of the
ring and, given its low mass, dynamical friction is unlikely to have played a
major role in its orbital evolution. We search cosmological simulations for
low-mass satellites in nearly-circular orbits and find that they result, almost
invariably, from orbital changes induced by collisions with more massive
satellites: the radius of the circular orbit thus traces the galactocentric
distance of the collision. Interestingly, the Sagittarius dwarf, one of the
most luminous satellites of the Milky Way, is in a polar orbit that crosses the
Galactic plane at roughly the same Galactocentric distance as Monoceros. We use
idealized simulations to demonstrate that an encounter with Sagittarius might
well have led to the circularization and subsequent tidal demise of the
progenitor of the Monoceros ring.
