The Spanish Flu pandemic of 1918 was deadly, resulting in the deaths of around 675,000 Americans and 50 million people worldwide. By the time it ended in 1920, it was responsible for more deaths than World War I, and may have killed between 3 and 6 percent of the global population.
The one thing it definitely isn't responsible for is ending World War II, which took place began in 1939 and ended in 1945. You wouldn't know this if you got your facts from the President of the United States, who told reporters yesterday that the 1917 [sic] pandemic was responsible for ending a war that started decades after the outbreak petered out.
“The closest thing [to Covid-19] is in 1917, they say, the great pandemic," Donald Trump said in the press conference. "It certainly was a terrible thing where they lost anywhere from 50 to 100 million people, probably ended the Second World War. All the soldiers were sick. It was a terrible situation."
This isn't the first historically questionable quote of this kind from the president, who on Independence Day last year praised the revolutionary armies for taking over airports from the British in the 1775–1783 war, despite airplanes being invented in 1903.
It's likely that Trump was referring to the First World War, rather than the Second, of course, but it's still a stretch to say the Spanish flu probably ended WWI, either. Influenza moved through troops like wildfire, killing over 45,000 American soldiers alone – a larger death toll than any single battle the US was involved in. Cramped conditions and movements of soldiers contributed to the spread of the virus around the world, according to the Centers for Disease Control and Prevention (CDC). So it certainly played a role in the last year of the war, but not significantly enough to say that it ended it, given all the other factors, such as large offensives by the Allies, the Russian Revolution, Germany stretching its forces too thinly in its attempt to occupy Ukraine, mutinies among Austria-Hungary troops, and the US itself joining the war.
"Although 1918 pandemic influenza might have claimed toward 100 000 fatalities among soldiers overall during the conflict and rendered millions ineffective, it remains unclear to this day whether the disease had an impact on the course of WWI," Peter C. Wever and Leo van Bergen concluded in a 2014 paper looking at the impact of the disease on the course of the war, published in Influenza and other respiratory diseases.
"While, for instance, commanding officers complained that the flu was affecting fighting strength, planned offensives had to be delayed, and the morale was further lowered, the effects of 1918 pandemic influenza in purely military terms were probably minimal, even during the second wave. This resulted from the extremely virulent nature of the virus, which came, killed, and moved on."
All sides suffered from the deadly virus, with some historians arguing that the Germans were hit hardest. Exact numbers are difficult to trace, as influenza was rarely recorded as the specific cause of death. Instead, records favored vague terms such as "disease contracted on the battlefield".
So while the Spanish Flu was certainly a major part of the last year of World War I, it's far from what ended it, let alone World War II, 20 years later.
"[The] 1918 pandemic influenza may have been of little significance militarily," Wever and Bergen concluded.
"It was a disaster of enormous magnitude from a purely human point of view."