  Several lines of evidence suggest the Milky Way underwent a major merger at
z~2 with a galaxy known as Gaia-Sausage-Enceladus (GSE). Here we use H3 Survey
data to argue that GSE entered the Galaxy on a retrograde orbit based on a
population of highly retrograde stars with chemistry similar to the largely
radial GSE debris. We present the first tailored, high-resolution N-body
simulations of the merger. From a grid of ~500 simulations we find a GSE with
$M_{*}=5\times10^{8}\ M_{\odot}, M_{\rm{DM}}=2\times10^{11} M_{\odot}$ (a 2.5:1
total mass merger) best matches the H3 data. This simulation shows the
retrograde GSE stars are stripped from its outer disk early in the merger
before the orbit loses significant angular momentum. Despite being selected
purely on angular momenta and radial distributions, this simulation reproduces
and explains the following empirical phenomena: (i) the elongated, triaxial
shape of the inner halo (axis ratios $10:7.9:4.5$), whose major axis is at
~35{\deg} to the plane and connects GSE's apocenters, (ii) the Hercules-Aquila
Cloud & the Virgo Overdensity, which arise due to apocenter pile-up, (iii) the
2 Gyr lag between the quenching of GSE and the truncation of the age
distribution of the in-situ halo, which tracks the 2 Gyr gap between the first
and final GSE pericenters. We make the following predictions: (i) the inner
halo has a "double-break" density profile with breaks at both ~15-18 kpc and 30
kpc, coincident with the GSE apocenters, (ii) the outer halo has retrograde
streams containing ~10% of GSE stars awaiting discovery at >30 kpc. The
retrograde (radial) GSE debris originates from its outer (inner) disk --
exploiting this trend we reconstruct the stellar metallicity gradient of GSE
($-0.04\pm0.01$ dex $r_{\rm{50}}^{-1}$). These simulations imply GSE delivered
~20% of the Milky Way's present-day dark matter and ~50% of its stellar halo.
(ABRIDGED)
