Which wartime innovation led to the formalization of the modern military PX (Post Exchange) retail system that now generates over $8 billion in annual sales?

The military exchange system is one of the oldest and largest retail operations in America, serving troops worldwide. These Product Exchange (PX) stores have evolved from simple sutler wagons following armies to modern retail giants with billions in annual sales. They provide tax-free goods to military personnel while generating profits that fund quality-of-life programs on bases. This poll tests your knowledge about the historical development of military retail operations that shaped modern military commerce.

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The Civil War 'sutler' system of civilian merchants who followed Union armies
World War I canteens established by the War Department in 1917
World War II Navy Ship's Service stores established in 1942
Vietnam War field PXs created to boost troop morale in 1965