  Pressure balance plays a central role in models of the interstellar medium
(ISM), but whether and how pressure balance is realized in a realistic
multiphase ISM is not yet well understood. We address this question using a set
of FIRE-2 cosmological zoom-in simulations of Milky Way-mass disk galaxies, in
which a multiphase ISM is self-consistently shaped by gravity, cooling, and
stellar feedback. We analyze how gravity determines the vertical pressure
profile as well as how the total ISM pressure is partitioned between different
phases and components (thermal, dispersion/turbulence, and bulk flows). We show
that, on average and consistent with previous more idealized simulations, the
total ISM pressure balances the weight of the overlying gas. Deviations from
vertical pressure balance increase with increasing galactocentric radius and
with decreasing averaging scale. The different phases are in rough total
pressure equilibrium with one another, but with large deviations from thermal
pressure equilibrium owing to kinetic support in the cold and warm phases,
which dominate the total pressure near the midplane. Bulk flows (e.g., inflows
and fountains) are important at a few disk scale heights, while thermal
pressure from hot gas dominates at larger heights. Overall, the total midplane
pressure is well-predicted by the weight of the disk gas, and we show that it
also scales linearly with the star formation rate surface density (Sigma_SFR).
These results support the notion that the Kennicutt-Schmidt relation arises
because Sigma_SFR and the gas surface density (Sigma_g) are connected via the
ISM midplane pressure.
