  Photometric surveys of M31's halo vividly illustrate the wreckage caused by
hierarchical galaxy formation. Several of M31's satellites are being disrupted
by M31's tidal field, among them M33 and And I, while other tidal structures
are the corpses of satellites already destroyed. The extent to which M31's
satellites have left battle scars upon it is unknown; to answer this we need
accurate orbits and masses of the perturbers. I focus here on M31's
150-kpc-long Giant Southern Stream (GSS) as an example of how these can be
determined even in the absence of a visible progenitor. Comparing N-body models
to photometric and spectroscopic data, I find this stream resulted from the
disruption of a large satellite galaxy by a close passage about 750 Myr ago.
The GSS is connected to several other debris structures in M31's halo. Bayesian
sampling of the simulations estimates the progenitor's initial mass as
log(Mstar/Msun) = 9.5 +- 0.2, showing it was one of the most massive Local
Group galaxies until quite recently. The stream model constrains M31's halo
mass to be (1.8+-0.5)x10^{12} Msun. While these small uncertainties neglect
several important degrees of freedom, they are likely to remain good even with
a more complete model. Future work on M31's satellites and streams will provide
independent constraints on M31's mass, and reveal the shared history of M31 and
its halo components.
