  The high-energy universe has revealed that energetic particles are ubiquitous
in the cosmos and play a vital role in the cultivation of cosmic environments
on all scales. Energetic particles in our own galaxy, galactic cosmic rays
(GCRs), engage in a complex interplay with the interstellar medium and magnetic
fields in the galaxy, giving rise to many of its key characteristics. This
White Paper is the first of a two-part series highlighting the most well-known
high-energy cosmic accelerators and contributions that MeV gamma-ray astronomy
will bring to understanding their energetic particle phenomena. The focus of
this white paper is galactic cosmic rays, supernova remnants, protostellar jets
and superbubbles, and colliding wind binaries.
