  One of the key mysteries of star formation is the origin of the stellar
initial mass function (IMF). The IMF is observed to be nearly universal in the
Milky Way and its satellites, and significant variations are only inferred in
extreme environments, such as the cores of massive elliptical galaxies. In this
work we present simulations from the STARFORGE project that are the first
cloud-scale RMHD simulations that follow individual stars and include all
relevant physical processes. The simulations include detailed gas
thermodynamics, as well as stellar feedback in the form of protostellar jets,
stellar radiation, winds and supernovae. In this work we focus on how stellar
radiation, winds and supernovae impact star-forming clouds. Radiative feedback
plays a major role in quenching star formation and disrupting the cloud,
however the IMF peak is predominantly set by protostellar jet physics. We find
the effect of stellar winds is minor, and supernovae occur too late}to affect
the IMF or quench star formation. We also investigate the effects of initial
conditions on the IMF. The IMF is insensitive to the initial turbulence, cloud
mass and cloud surface density, even though these parameters significantly
shape the star formation history of the cloud, including the final star
formation efficiency. The characteristic stellar mass depends weakly on
metallicity and the interstellar radiation field. Finally, while turbulent
driving and the level of magnetization strongly influences the star formation
history, they only influence the high-mass slope of the IMF.
