We study the formation histories and present-day structure of satellite galaxies formed in a high-resolution hydrodynamic simulation of a Milky Way like galaxy. The simulated satellites span nearly four orders of magnitude in luminosity but have a very similar mass within their inner 600 pc, ∼3 × 107 M⊙, with very little scatter. This result is in agreement with the recent measurements for dwarf spheroidal galaxies (dSphs) in the Milky Way by Strigari et al. In our simulations, a preferred mass scale arises naturally from the effects of the early re-ionization of gas. These impose a sharp threshold of ∼12 km s−1 on the circular velocity of haloes which can cool gas and make stars. At the present day, subhaloes that host satellites as luminous as the classical Milky Way dwarfs (LV≥ 2.6 × 105 L⊙) have typically grown to have circular velocities of ≳20 km s−1. There are, however, subhaloes with similar circular velocities today which were, nevertheless, below threshold at re-ionization and thus remain dark. Star formation in above-threshold haloes is truncated when the halo is accreted into the main galaxy progenitor. Thus, many properties of today's dwarf satellites such as their luminosity and star formation rate are set by their accretion time.