A theory of solid-state fusion based on the interaction between D+ and 4He++ ion band states within a host lattice is presented. Formation of ion band-state deuterium is thermodynamically favored when lattice strain energy is greater than the incremental chemical potential of the band state. The key fusion step is a coalescence fluctuation that converts a two-fold occupation state of electrostatic zero-point-motion size into a state of nuclear dimensions. Rates are calculated using the Fermi Golden Rule. Fusion energy is shared between band-state members and subsequently transferred to the lattice.