Which wartime retail innovation led to the modern military PX (Post Exchange) system's dramatic expansion and standardization?

The Post Exchange (PX) system has been a crucial retail innovation for military morale since its formal establishment. These military department stores provide tax-free goods to service members worldwide and have evolved significantly over time. Test your knowledge about this unique retail strategy that has supported troops through multiple conflicts while becoming an important cultural touchstone for military families.

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World War II's Ship's Service stores and Army Exchange Service merger
Vietnam War's introduction of AAFES catalog ordering system
Korean War's implementation of the first overseas PX facilities
Gulf War's development of combat zone mini-exchanges