Yuletide cheer was in short supply in Washington, D.C., on the day after Christmas in 1941.

Less than three weeks after the Japanese attack on Pearl Harbor pulled the United States into World War II, the American capital was a city on edge. Uniformed soldiers patrolled the streets and manned anti-aircraft guns atop government buildings. Under the direction of Secret Service Chief Frank Wilson, blackout shutters had been installed in the White House and its roof camouflaged. A miniature mountain of dirt dumped near the Washington Monument bore the remnants of a 761-foot-long escape tunnel that had been hastily burrowed from the White House to the Treasury Building. A gas mask dangled from President Franklin D. Roosevelt’s wheelchair, waiting to be worn at any second.

“The fear of an attack in Washington, D.C., was really palpable, and the big fears on the East Coast were German bombs and sabotage” says Stephen Puleo, author of the new book “American Treasures: The Secret Efforts to Save the Declaration of Independence, the Constitution and the Gettysburg Address.” “America’s ‘Pearl Harbor Christmas’ is this tenuous, uncertain time.”

As another tense evening began in the capital on December 26, 1941, Secret Service agents suddenly cleared the public from a platform in Union Station and escorted four plain-wrapped cases onto a Baltimore & Ohio National Limited bound for Louisville. The parcels looked nondescript, but hidden inside was America’s soul.

The packing list that itemized the contents of the four cases shipped by the Library of Congress may never be rivaled. Inside three cartons were the Gettysburg Address, Lincoln’s Second Inaugural Address, the Articles of Confederation, three volumes of the Gutenberg Bible and a copy of the Magna Carta entrusted to the Library of Congress for safekeeping after its exhibition at the 1939 New York World’s Fair. The fourth case contained the most priceless cargo of all—the original parchment versions of the U.S. Constitution and Declaration of Independence. As the train pulled out of the station, America’s treasured possessions disappeared into the night, bound for a secret hiding spot 600 miles away.

The plan to safeguard America’s most precious documents began 14 months earlier as war raged across Europe in the fall of 1940, Puleo tells HISTORY. “Across Europe the Nazis were destroying millions of books and manuscripts,” he says. German incendiary bombs had incinerated more than a dozen British libraries, and during the London Blitz, a bomb dropped on the British Museum had seriously damaged hundreds of books that belonged to King George III.

Archibald MacLeish, the man handpicked by Roosevelt to head the Library of Congress, feared a similar fate for the documents entrusted to him, including the U.S. Constitution and the Declaration of Independence. (Those founding documents did not move to the National Archives until 1952.) “MacLeish puts the Library of Congress on a war footing in the same way that industrial plants and assembly lines were,” Puleo says. “He tells his staff to assess, catalog and record documents they thought were unique and indispensable to American democracy.”

For 10 weeks during the spring of 1941, 700 Library of Congress staff members and volunteers worked more than 10,000 hours to identify and pack nearly 5,000 boxes of irreplaceable documents, music, maps, rare books and artifacts that could be relocated to a more secure location in the event of war. A bureaucratic turf battle precluded the use of the newly completed National Archives building, which was built to withstand a conventional bombing, and Congress had refused to fund construction of a bombproof bunker in between the Library of Congress and Supreme Court.

MacLeish asked Treasury Secretary Henry Morgenthau Jr. whether space was available inside the newly built gold bullion repository vault at Kentucky’s Fort Knox. With permanent military protection and underground steel and concrete bunkers capable of withstanding enemy attack, the vault was perhaps the most secure location in the country. However, it was also teeming with tons of gold bullion that had been moved there as fears of war strengthened. Morgenthau told MacLeish that he had available space—but only 60 cubic feet, about the size of a standing freezer, Puleo says.

That would do for the library’s most important possessions but not 5,000 boxes, so employees scouted additional repositories that would be beyond the likely range of enemy bombers and capable of preserving the fragile records. “They had to be somewhat inland but not too far inland because library staff needed to reach them to inspect the documents,” Puleo says. “They also needed to be protected from humidity, mites, pests and potential water damage.” After assessing more than 30 sites, the Library of Congress chose the University of Virginia, Washington and Lee University and the Virginia Military Institute to house the boxes. (Denison University in Ohio would be added to the roster after the war began.)

When war finally came to America, MacLeish was prepared. As British Prime Minister Winston Churchill delivered a joint address to Congress on December 26, 1941, few knew of the secret operation taking place directly across the street. Inside the Library of Congress, MacLeish personally packed up what he called “the documentary history of freedom in our world.” First he sealed the documents bound for Fort Knox in a specially prepared bronze container that had been heated for six hours to a temperature of around 90 degrees in order to “drive off any moisture.” The container was then embedded in mineral wool and placed in a larger wooden case that was secured with screws, wires and padlocks and sealed with lead. The four plain parcels were then loaded into a waiting armored truck and brought to Union Station for the rail journey to Kentucky. “I suppose it is quite literally true that no shipment of a value even remotely approaching the value of this shipment was ever made in this country,” MacLeish wrote.

Over the ensuing weeks, thousands of boxes containing other priceless documents such as George Washington’s diaries and the Journals of the Continental Congress were secretly removed from the Library of Congress and delivered by truck to the university repositories. In spite of the number of people involved—from the librarians to truck drivers—the operation remained clandestine for the entirety of the war. “Everyone maintained the secrecy, which I think would be impossible today,” Puleo says.

Outside of a brief cameo by the Declaration of Independence at the Jefferson Memorial’s dedication in April 1943, America’s founding documents remained absent from the nation’s capital for nearly three years until the Allies finally gained the upper hand in the war in the months following D-Day. With fears of a possible Nazi attack on the American mainland abated, most of the 5,000 boxes were returned home to the Library of Congress in the summer of 1944. The only missing items, according to Puleo, were two notebooks that had belonged to the American poet Walt Whitman, one of which remains unaccounted for seven decades later.

On October 1, 1944, the U.S. Constitution and Declaration of Independence returned to public view inside the Library of Congress. Before the doors opened, MacLeish addressed the Marine guard of honor tasked with protecting the documents. “Our nation differs from all others in this—that it was not created by geographic or by racial accident, but by the free choice of the human spirit, conceived and founded by men who chose to live under one form of government rather than under another,” he said. “The sheets of vellum and the leaves of ancient paper in those cases which you guard are the very sheets and leaves on which [our] form of government [was] brought to being. Nothing that men have ever made surpasses them.”