Which wartime innovation fundamentally transformed military retail by introducing the modern Post Exchange (PX) system that still serves military personnel today?

The Post Exchange (PX) system represents one of military history's most enduring retail innovations. These stores have evolved from simple sutlers' tents during the Civil War to global retail networks serving military personnel worldwide. This trivia question explores how this unique military retail institution transformed during a pivotal moment in military history, blending retail innovation with military necessity.

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World War II's Ship's Service stores on aircraft carriers
World War I's establishment of the Army Exchange Service in 1917
Vietnam War's introduction of mobile PX units for remote bases
Korean War's integration of civilian retail partnerships with military stores