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The Great Andromeda Galaxy (M31) is the nexus of the near-far galaxyevolution connection and a principal data point for near-field cosmology. Dueto its proximity (780 kpc), M31 can be resolved into individual stars like theMilky Way (MW). Unlike the MW, we have the advantage of a global view of M31,enabling M31 to be observed with techniques that also apply to more distantgalaxies. Moreover, recent evidence suggests that M31 may have survived a majormerger within the last several Gyr, shaping the morphology of its stellar haloand triggering a starburst, while leaving the stellar disk largely intact. TheMW and M31 thus provide complementary opportunities for in-depth studies of thedisks, halos, and satellites of L* galaxies. Our understanding of the M31 system will be transformed in the 2020s if theyinclude wide field facilities for both photometry (HST-like sensitivity andresolution) and spectroscopy (10-m class telescope, >1 sq. deg. field, highlymultiplexed, R~ 3000 to 6000). We focus here on the power of these facilitiesto constrain the past, present, and future merger history of M31, viachemo-dynamical analyses and star formation histories of phase-mixed starsaccreted at early times, as well as stars in surviving tidal debris features,M31's extended disk, and intact satellite galaxies that will eventually betidally incorporated into the halo. This will yield an unprecedented view ofthe hierarchical formation of the M31 system and the subhalos that built itinto the L* galaxy we observe today.