What was the fundamental principle explored during WWII that laid the conceptual groundwork for RFID?

Using reflected radio waves to identify aircraft (passive transponder system).

The conceptual groundwork for the technology that would become RFID originated from innovations during World War II, specifically concerning military needs. Researchers explored using reflected radio waves as a method to passively identify Allied aircraft, aiming to prevent friendly fire incidents. This early approach involved transmitting an interrogation signal that was then reflected back by the target, confirming its presence and identity. Although this system was not the commercial RFID known today, it established the core physics principle—remote identification via radio wave reflection—that provided the necessary theoretical basis for later researchers who aimed to commercialize remote identification methods.