  Supernovae (SNe) dominate the energy and momentum budget of stellar feedback,
but the efficiency with which they couple to the interstellar medium (ISM)
depends strongly on how effectively early, pre-SN feedback clears dense gas
from star-forming regions. There are observational constraints on the
magnitudes and timescales of early stellar feedback in low ISM pressure
environments, yet no such constraints exist for more cosmologically typical
high ISM pressure environments. In this paper, we determine the mechanisms
dominating the expansion of HII regions as a function of size-scale and
evolutionary time within the high-pressure ($P/k_\rm{B}$~$10^{7-8}$K cm$^{-3}$)
environment in the inner 100pc of the Milky Way. We calculate the thermal
pressure from the warm ionised ($P_\rm{HII}$; 10$^{4}$K) gas, direct radiation
pressure ($P_\rm{dir}$), and dust processed radiation pressure ($P_\rm{IR}$).
We find that (1) $P_\rm{dir}$ dominates the expansion on small scales and at
early times (0.01-0.1pc; $<$0.1Myr); (2) the expansion is driven by
$P_\rm{HII}$ on large scales at later evolutionary stages ($>0.1$pc; $>1$Myr);
(3) during the first ~1Myr of growth, but not thereafter, either $P_{\rm IR}$
or stellar wind pressure likely make a comparable contribution. Despite the
high confining pressure of the environment, natal star-forming gas is
efficiently cleared to radii of several pc within ~2Myr, i.e. before the first
SNe explode. This `pre-processing' means that subsequent SNe will explode into
low density gas, so their energy and momentum will efficiently couple to the
ISM. We find the HII regions expand to a radius of 3pc, at which point they
have internal pressures equal with the surrounding external pressure. A
comparison with HII regions in lower pressure environments shows that the
maximum size of all HII regions is set by pressure equilibrium with the ambient
ISM.
