  The numerous streams in the M31 halo are currently assumed to be due to
multiple minor mergers. Here we use the GADGET2 simulation code to test whether
M31 could have experienced a major merger in its past history. It results that
a 3+/-0.5:1 gaseous rich merger with r(per)=25+/-5 kpc and a polar orbit can
explain many properties of M31 and of its halo. The interaction and the fusion
may have begun 8.75+/-0.35 Gyr and 5.5 +/-0.5 Gyr ago, respectively. With an
almost quiescent star formation history before the fusion we retrieve fractions
of bulge, thin and thick disks as well as relative fractions of intermediate
age and old stars in both the thick disk and the Giant Stream. The Giant Stream
is caused by returning stars from a tidal tail previously stripped from the
satellite prior to the fusion. These returning stars are trapped into
elliptical orbits or loops for almost a Hubble time period. Large loops are
also predicted and they scale rather well with the recently discovered features
in the M31 outskirts. We demonstrate that a single merger could explain
first-order (intensity and size), morphological and kinematical properties of
the disk, thick disk, bulge and streams in the halo of M31, as well as the
distribution of stellar ages, and perhaps metallicities. It challenges
scenarios assuming one minor merger per feature in the disk (10 kpc ring) or at
the outskirts (numerous streams & thick disk). Further constraints will help to
properly evaluate the impact of such a major event to the Local Group.
