Which wartime development led to the formal establishment of the modern military Post Exchange (PX) system that revolutionized retail strategy for military personnel?

Military exchanges (PXs/BXs) have a fascinating history dating back to the 19th century, evolving from sutlers who followed armies to today's modern retail operations serving military personnel worldwide. These distinctive retail environments represent a unique sector where commerce meets service to military communities. Test your knowledge about how military retail operations have evolved through wartime and peace!

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The Spanish-American War (1898), when the War Department established regulated exchanges to replace civilian sutlers
World War I (1917), when the Army created the first centralized commissary system for deployed troops
The Civil War (1861-1865), when Lincoln authorized official government-run stores at Union Army camps
World War II (1941-1945), when the DoD consolidated separate service exchanges into a unified retail operation