  Recent observations of our neighbouring galaxy M31 have revealed that its
disk was shaped by widespread events. The evidence for this includes the high
dispersion ($V/\sigma$ $\le$ 3) of stars older than 2 Gyr, and a global star
formation episode, 2-4 Gyr ago. Using the modern hydrodynamical code, GIZMO, we
have performed 300 high-resolution simulations to explore the extent to which
these observed properties can be explained by a single merger. We find that the
observed M31 disk resembles models having experienced a 4:1 merger, in which
the nuclei coalesced 1.8-3 Gyr ago, and where the first passage took place 7 to
10 Gyr ago at a large pericentre distance (32 kpc). We also show that within a
family of orbital parameters, the Giant Stream (GS) can be formed with various
merger mass-ratios, from 2:1 to 300:1. A recent major merger may be the only
way to create the very unusual age-dispersion relation in the disk. It
reproduces and explains the long-lived 10 kpc ring, the widespread and recent
star formation event, the absence of a remnant of the GS progenitor, the
apparent complexity of the 3D spatial distribution of the GS, the NE and G
Clumps and their formation process, and the observed slope of the halo profile.
These modelling successes lead us to propose that the bulk of the substructure
in the M31 halo, as well as the complexity of the inner galaxy, may be
attributable to a single major interaction with a galaxy that has now fully
coalesced with Andromeda.
