The US definitely had a notion of quantum physics prior to WWII. Feynman got his PhD at Princeton in 1942 in Quantum Physics so I would assume that John Wheeler had some familiarity with the topic back then. I would mention that the most significant result of quantum mechanics is solid state transistors, and Shockley was awarded a phd for quantum mechanical applications back in the 30s.

Michelson and his experiments on the aether not existing were enormously influential to theoretical physics. “No notion” is incorrect, they had numerous home grown talents in physics, on top of the huge influx of talent from 1930s immigration of European scientists.

The USA being a beacon of hope and enlightenment in those days stands in stark contrast to the isolationist, anti-intellectual, anti-research, and frankly xenophobic policies pursued by the current admin, courts, and congress.