What was the deadliest day in US history?

If you think of the deadliest day in the United States' history, your mind is probably drawn to the terrorist attacks of September 11, the calamity that followed Japan's strike on Pearl Harbor or perhaps a battle from the Civil War. Or maybe you think of more recent days during the COVID-19 pandemic. 

The answer to the question of what was the deadliest day, it turns out, isn't straightforward. But when you take the death rate into account, it's likely none of the events mentioned above.

To put contemporary U.S. deaths in perspective, before COVID-19 began circulating in late 2019, approximately 7,700 people died every day in the U.S. for a multitude of different reasons, including things like car accidents and heart disease, said J. David Hacker, a demographic historian at the University of Minnesota.

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The deadliest day in America's history is hard to pinpoint because, for one thing, America's population has grown considerably, from a mere 4 million in 1790 to in excess of 332 million today, Hacker said. So, comparing the absolute number of deaths from yesteryear with today is like comparing apples with oranges.

"Of course there are more overall deaths in a typical day today than there were in 1790, despite the fact that the death rate — deaths divided by the population — was undoubtedly much higher in 1790," Hacker told Live Science. But even if we decide that the death rate is the fairer way to make comparisons across the centuries, finding an answer to the "deadliest day' question is still more complicated than you might think.

"The deadliest day comparisons I've seen rely on different measures," Hacker said. If we're looking at a single attack or event, do we discount the people who also died on that day, but from other causes? Or do we include them? There's not much of a consensus among historians, and, on top of that, death records nationwide from 1776 to now are lacking, Hacker said.

That said, we can make a few educated guesses. "If it's just the total number of deaths in one day from a specific event on a specific day, I think nothing comes close to the Galveston Hurricane on Sept. 8, 1900," Hacker said. The hurricane, which struck Texas as a Category 4 hurricane with winds from 130 to 156 mph (209 to 251 km/h), is also known as "The Great Storm of 1900," and is often described as the deadliest natural disaster in U.S. history, according to the National Oceanic and Atmospheric Administration (NOAA). Between 8,000 and 12,000 people were killed during the hurricane, NOAA said in a 2011 report. Back in 1900, roughly 3,500 people died each day, on average, said Hacker, so the storm was an especially lethal event. 

Meanwhile, the Civil War, waged from 1861-1865, was an especially bloody time. It's estimated that 750,000 soldiers perished from injury and disease, according to a 2011 study in the journal Civil War History. And so, it's not too surprising that another event worthy of mention is the 1862 Battle of Antietam, which thwarted the Confederate invasion of Maryland and saw an estimated 3,650 soldiers killed from both sides. 

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But here again we come across data problems — not all who fought in the battle and died did so on the day of the battle itself. "Men wounded in the one-day battle may have suffered for weeks or months before finally dying, and are likely not part of the estimate," Hacker said. "Counting Civil War deaths is not an exact science." 

Hacker roughly estimates that about 2,500 other people died in the U.S. due to other (non-war-related) causes on the same day as the Battle of Antietam. That means the battle dead more than doubled the death rate of that day, making it a pretty deadly day by anyone's reckoning. The tally was higher for the Battle of Gettysburg in July 1863 — where more than 7,000 soldiers were killed — but that happened over the course of three days, he said. 

Putting violence aside, the Spanish flu was another particularly lethal period. "About 6,000 people died each day during October 1918 from influenza, on average." Hacker said. If we had better data from that time, it might have been possible to say that the Spanish flu was responsible for the deadliest day in U.S. history because some days likely eclipsed that 6,000 figure. "If we knew the one-day peak number of deaths from influenza, alas we don't, and added that to the daily total from other causes," Hacker said, "then perhaps the deadliest day in U.S. history from all causes or events was in October 1918." However, we don't have the records to back that up, so it’s still possible that the Galveston Hurricane was a bigger killer; in the end it comes down to a judgement call more than an indisputable fact.

What about COVID-19? During the worst days of the pandemic in February 2021, approximately 3,300 people were dying each day from the novel coronavirus, which exceeds the close to 3,000 people who died on Sept. 11, 2001, when terrorists hijacked planes, which crashed into the Twin Towers in New York City, the Pentagon in Arlington, Virginia, and in a field in Pennsylvania. 

If we add that COVID-19 figure to the approximate 7,700 other deaths that happen, on average, every day, we can say that close to 11,000 people in America were dying per day during the worst days of February 2021. While not detracting from the very real tragedy of COVID-19, the population in 1918 was one-third of what it is today, and so for that reason Hacker ranks the Spanish flu above COVID-19, even though in absolute numbers, the COVID-19 pandemic may have killed more people on its deadliest day. 

"For my money, the deadliest day in U.S. history was probably one of those days in October 1918," when you take the death rate into account, he said.

Originally published on Live Science.