  We present the color-magnitude diagrams and star formation histories (SFHs)
of seven ultra-faint dwarf galaxies: Horologium 1, Hydra 2, Phoenix 2,
Reticulum 2, Sagittarius 2, Triangulum 2, and Tucana 2, derived from
high-precision Hubble Space Telescope photometry. We find that the SFH of each
galaxy is consistent with them having created at least 80% of the stellar mass
by $z\sim6$. For all galaxies, we find quenching times older than 11.5 Gyr ago,
compatible with the scenario in which reionization suppresses the star
formation of small dark matter halos. However, our analysis also reveals some
differences in the SFHs of candidate Magellanic Cloud satellites, i.e.,
galaxies that are likely satellites of the Large Magellanic Cloud and that
entered the Milky Way potential only recently. Indeed, Magellanic satellites
show quenching times about 600 Myr more recent with respect to those of other
Milky Way satellites, on average, even though the respective timings are still
compatible within the errors. This finding is consistent with theoretical
models that suggest that satellites' SFHs may depend on their host environment
at early times, although we caution that within the error bars all galaxies in
our sample are consistent with being quenched at a single epoch.
