Background and purpose Construction of the Pentagon began on September 11, 1941, as the United States prepared for expanded involvement in World War II. The War Department sought to consolidate scattered offices into a single, secure, and efficient headquarters near the capital. The building’s distinctive five-sided plan, conceived in part to fit the irregular site and to minimize walking distances between offices, resulted in a sprawling complex that could house tens of thousands of employees. Construction and scale The Pentagon was built rapidly using wartime materials and labor, with an emphasis on speed and economy. Despite the constraints of the era, the building’s design incorporated durable materials and straightforward construction techniques. By early 1943 the primary structure was essentially complete, and occupancy of many offices had already begun prior to the formal dedication. At the time, it was among the largest office buildings in the world by floor area, with miles of corridors linking concentric rings of workspace around an open central courtyard. The dedication, March 20, 1943 The formal dedication took place on March 20, 1943. The ceremony marked the official opening of the building as the headquarters for the War Department. Photographs and contemporary news accounts focused on the symbolic significance of consolidating military administration in a single modern facility during a time of global conflict. The dedication underscored the U.S. government’s emphasis on organizational efficiency and coordination of the nation’s wartime resources. Operation and legacy Following the dedication, the Pentagon rapidly became the nerve center for U.S. military planning and logistics through the remainder of World War II and in subsequent decades after the War Department reorganized into the Department of Defense in 1947. Its scale, centralization of offices, and secure design influenced later military and large-scale administrative facilities worldwide. The building has undergone renovations, expansions of services, and security adaptations over the decades, but its 1943 dedication remains a key milestone in its history. Notes on sources and interpretation Contemporary newspaper reports, government records, and architectural histories document the timeline: construction beginning in 1941, phased occupancy before 1943, and the formal dedication on March 20, 1943. Some descriptions emphasize that portions of the building were already in use before the dedication; the March 20 ceremony therefore commemorated formal opening rather than the literal first use of every office. Where accounts differ, this summary reflects the widely accepted date of dedication.