  Magnetic fields may play a crucial role in setting the initial conditions of
massive star and star cluster formation. To investigate this, we report
SOFIA-HAWC+ $214\:\mu$m observations of polarized thermal dust emission and
high-resolution GBT-Argus C$^{18}$O(1-0) observations toward the massive
Infrared Dark Cloud (IRDC) G28.37+0.07. Considering the local dispersion of
$B$-field orientations, we produce a map of $B$-field strength of the IRDC,
which exhibits values between $\sim0.03 - 1\:$mG based on a refined
Davis-Chandrasekhar-Fermi (r-DCF) method proposed by Skalidis \& Tassis.
Comparing to a map of inferred density, the IRDC exhibits a $B-n$ relation with
a power law index of $0.51\pm0.02$, which is consistent with a scenario of
magnetically-regulated anisotropic collapse. Consideration of the mass-to-flux
ratio map indicates that magnetic fields are dynamically important in most
regions of the IRDC. A virial analysis of a sample of massive, dense cores in
the IRDC, including evaluation of magnetic and kinetic internal and surface
terms, indicates consistency with virial equilibrium, sub-Alfv\'enic conditions
and a dominant role for $B-$fields in regulating collapse. A clear alignment of
magnetic field morphology with direction of steepest column density gradient is
also detected. However, there is no preferred orientation of protostellar
outflow directions with the $B-$field. Overall, these results indicate that
magnetic fields play a crucial role in regulating massive star and star cluster
formation and so need to be accounted for in theoretical models of these
processes.
