During the 1933–1945 window, molecular thermodynamics shifted toward integrating statistical mechanics with condensed-phase behavior, yielding a cohesive molecular‑level framework for phase phenomena. The era produced region‑based dielectric treatments, rigorous phase‑integral formalisms, lattice models for polymers, and universal state descriptions that connect microscopic interactions with macroscopic properties. As a result, researchers established a practical, predictive paradigm that linked internal molecular motion, local structure, and phase behavior across diverse substances.