This is the first of a sequence of papers that addresses the nonperturbative origin of the central and spin-dependent forces between quarks. Its main thrust is a focus on meson spectroscopy in the center-of-mass frame. We suggest a novel “dense instanton ensemble” model for the QCD vacuum, to explain the interquark forces in mesons, from quarkonia to heavy-light and light-light ones. The sequels will show how to export these interactions to the light front, and derive the corresponding Hamiltonians, mesonic, and b…