  As one of the most massive Milky Way satellites, the Sagittarius dwarf galaxy
has played an important role in shaping the Galactic disk and stellar halo
morphologies. The disruption of Sagittarius over several close-in passages has
populated the halo of our Galaxy with large-scale tidal streams and offers a
unique diagnostic tool for measuring its gravitational potential. Here we test
different progenitor mass models for the Milky Way and Sagittarius by modeling
the full infall of the satellite. We constrain the mass of the Galaxy based on
the observed orbital parameters and multiple tidal streams of Sagittarius. Our
semi-analytic modeling of the orbital dynamics agrees with full $N$-body
simulations, and favors low values for the Milky Way mass, $\lesssim
10^{12}M_\odot$. This conclusion eases the tension between $\Lambda$CDM and the
observed parameters of the Milky Way satellites.
