While Allied troops were fighting World War II in the Pacific, the U.S. homefront was defending Washington, D.C. from the worst flood it’d ever seen.

“Spare no effort or expense to protect the Capital,” President Franklin D. Roosevelt told officials during the flood of October 1942, according to a Washington Post article. As the waters surged inland from the Potomac River, 800 soldiers and 300 civilians feverishly stuffed sandbags and built a barrier to prevent the flood from reaching downtown federal buildings like the White House.

The flood, which covered the National Mall so thoroughly that the newly-built Jefferson Memorial looked like a little island, would be the worst to ever engulf the nation’s capital, according to the National Oceanic and Atmospheric Administration (NOAA). It also caused the Anacostia River to overflow, submerging the Navy Yard in the Southeast.

The over 1,000 soldiers and civilians “raised a half-mile-long, 6-foot sandbag levee on the north bank of the Potomac in six hours,” reported LIFE magazine in 1942. “As the water crept up, inch by inch, bulldozers were thrown into the fight and the entire area around the Navy Building became a scene of fevered activity,” reported The Washington Post at the time.

The flood was brought on by torrential rainfall likely related to a southern tropical storm. In D.C.’s tidal zone, the flood crest was 17.7 feet, nearly a half-foot higher than the crest during D.C.’s second-worst flood in March 1936, according to the National Weather Service.

The 1942 rains didn’t just affect D.C. They also flooded parts of Maryland, Virginia and West Virginia. LIFE reported at the time that the flooding “completely isolated” Fredericksburg, Virginia. It drove 1,500 people from their homes, killed more than a dozen people and contaminated the water supply.

In the D.C. area, the flood caused deaths and evacuations as well. John E. Buell, the chief of the Volunteer Fire Department in Bethesda, Maryland—a suburb of D.C.—died while trying to tow a car out of the water. Many people along the Potomac evacuated their homes. Some who’d survived the 1936 storm insisted on staying, which was extremely dangerous.

The National Mall is particularly vulnerable to flooding because it contains some of the lowest elevation points in the city and lies near the Potomac River (still, it was never a swamp, as the pervasive myth goes). After the 1936 flood, Congress passed the Flood Control Act of 1936 to prevent flooding on the Mall.

Under this act, the Army Corps of Engineers built a levee in 1939 between the Lincoln Memorial and the Washington Monument using landfill from the Capitol Reflecting Pool. But the levee had a gap at 17th Street NW, and according to a 2008 report by the National Capital Planning Commission, the U.S. removed “a considerable portion of the levee” for Navy Department construction during WWII.

After the war, Congress passed the Flood Control Act of 1946 to improve the levee and prevent future floods like the one in 1942. However, according to the 2008 report, the Army Corps of Engineers still hadn’t made the improvements because Congress never funded them. In 2014, the corps was finally able to update the levee.

This doesn’t mean D.C. is completely prepared for future storms. Rolling Stone reported in September 2017 that “prominent experts believe one of the most vulnerable targets” of hurricanes is Washington, D.C. A devastating storm could “paralyze many of the agencies that operate and defend the nation, raising the specter of national-security threats”—a fate that the country thankfully avoided during WWII.