  We use high-resolution cosmological hydrodynamical simulations of Milky
Way-sized galaxies with varying supernovae feedback strengths and merger
histories to investigate the formation of their gaseous halos and especially
their hot ($>10^6$~K) X-ray luminous coronae. Our simulations predict the
presence of significant hot gas in the halos as early as $z=3-4$, well before
the halos ought to be able to sustain hot mode accretion in the conventional
picture. The nascent coronae grow inside-out and initially do so primarily as a
result of outflows from the central galaxies powered by merger-induced shock
heating and strong supernovae feedback, both of which are elemental features of
today's successful galaxy formation models. Furthermore, the outflows and the
forming coronae also accelerate the transition from cold to hot mode accretion
by contributing to the conditions for sustaining stable accretion shocks. They
also disrupt the filamentary streams funneling cold gas onto the central
galaxies by causing their mouths to fray into a broad delta, detach from the
galaxies, and be pushed away to larger radii. And even though at early times
the filaments repeatedly re-form, the hot gas and the outflows act to weaken
the filaments and accelerate their ultimate disruption. Although galactic
outflows are generally thought of as ejective feedback, we find that their
action on the filaments suggests a preventive role as well.
