  The Milky Way's last significant merger, the Gaia Enceladus/Sausage (GES), is
thought to have taken place between 8-11 Gyr ago. Recent studies in the
literature suggest that the bar of the Milky Way is rather old, indicating that
it formed at a similar epoch to the GES merger. We investigate the possible
link between these events using one of the Auriga cosmological simulations
which has salient features in common with the Milky Way, including a last
significant merger with kinematic signatures resembling that of the GES. In
this simulation, the GES-like merger event triggers tidal forces on the disc,
gas inflows and a burst of star formation, with the formation of a bar occuring
within 1 Gyr of the first pericentre. To highlight the effects of the merger,
we rerun the simulation from z=4 with the progenitors of the GES-like galaxy
removed well before the merger time. The consequence is a delay in bar
formation by around 2 Gyr, and this new bar forms without any significant
external perturbers. We conclude that this Milky Way-like simulation shows a
route to the real Milky Way's bar forming around the epoch of the GES merger
due to tidal forces on its first pericentre. We explore all Auriga galaxies
with GES-like merger events, and find that those with stellar mass ratios below
10% form bars within 1 Gyr of the merger, while bar formation is delayed in the
more massive merger scenarios. These include the 4 oldest bars in the
simulation suite. Lastly, we note some later morphological differences between
the disc of the original simulation and our rerun, in particular that the
latter does not grow radially for the final 7 Gyr. Our study suggests that the
GES may therefore be responsible for the formation of the Milky Way's bar, as
well as for the build-up of its extended disc.
