  Observational studies indicate that the intergalactic medium (IGM) is highly
ionized up to redshifts just over 6. A number of models have been developed to
describe the process of reionization and the effects of the ionizing photons
from the first luminous objects. In this paper, we study the impact of an X-ray
background, such as high-energy photons from early quasars, on the temperature
and ionization of the IGM prior to reionization, before the fully ionized
bubbles associated with individual sources have overlapped. X-rays, which have
large mean free paths relative to EUV photons, and their photoelectrons can
have significant effects on the thermal and ionization balance. We find that
hydrogen ionization is dominated by the X-ray photoionization of neutral helium
and the resulting secondary electrons. Thus, the IGM may have been warm and
weakly ionized prior to full reionization. We examine several related
consequences, including the filtering of the baryonic Jeans mass scale,
signatures in the cosmic microwave background, and the H$^{-}$-catalyzed
production of molecular hydrogen.
