While there have been significant advances in our understanding of impulsive energy release at the Sun through the combination of RHESSI X-ray observations and SDO/AIA EUV observations, there is a clear science need for significantly improved X-ray and EUV observations. These new observations must capture the full range of emission in flares and CMEs (e.g., faint coronal sources near bright chromospheric sources), connect the intricate evolution of energy release with dynamic changes in the configuration of plasma structures, and identify the signatures of impulsive energy release in even the quiescent Sun. The Fundamentals of Impulsive Energy Release in the Corona Explorer (FIERCE) MIDEX mission concept makes these observations by combining the two instruments previously proposed on the FOXSI SMEX mission concept – a focusing hard X-ray spectroscopic imager and a soft X-ray spectrometer – with a high-resolution EUV imager that will not saturate for even intense flares. All instruments observe at high cadence to capture the initiation of solar transient events and the fine time structure within events. FIERCE would launch in mid-2025, near the peak of the next solar cycle, which is also well timed with perihelions of Parker Solar Probe and Solar Orbiter. The goals of FIERCE include improving our understanding of the genesis of space-weather events, and FIERCE obserations would vitally complement observations by other observatories both near Earth and elsewhere in the heliosphere.