The mighty USA have long-owned Olympic swimming, collecting 30 medals, including 11 gold, to top the table at the pandemic-delayed Tokyo Games, ahead of Australia with nine gold and 20 in total.
They are unbeaten on the gold medal front since 1988, but Australia stunned them by claiming more titles at the world championships last year to set up a blockbuster showdown over an intense nine days in La Defense Arena.
Adding spice, the US contested that they won the duel at the worlds in Fukuoka, claiming they were best courtesy of their higher overall medal count.
That saw recently-retired Australian legend Cate Campbell brand them "sore losers", which prompted 23-time Olympic gold medallist Michael Phelps to encourage his compatriots to "make them eat every word" in France.
Australian head coach Rohan Taylor attempted to dial down the rivalry, and expectations.
"I think the Americans are the standard of the sport," he said last week. "As a whole, in every event, they'll have somebody who can get on the podium. We want to be as good as them."
The decorated Ledecky, a seven-time gold medallist, is still the woman to beat in the 800m and the 1500m freestyle, but has lost her invincibility over 400m.
Six swimming world records tumbled in Tokyo and the first in Paris could come on the opening night with Ledecky attempting to reclaim her 400m crown against Australia's Ariarne Titmus and Canada's Summer McIntosh.
Known as "The Terminator", Titmus beat Ledecky in a thrilling Tokyo final then broke the American's long-standing world record in 2022, only for McIntosh to better the mark last year.
The Australian hit back to reclaim the world record months later and swam the second fastest time in history last month.
"I don't think about the result or the magnitude of the Olympic Games," said Titmus. "I just think about putting together eight great laps."
Titmus will also race the 200m freestyle -- as world record holder -- and 800m where she will also face Ledecky in the American's quest for a fourth straight title.
More world records will be on the line in the women's 100m and 200m backstroke, with Australia's Kaylee McKeown and American Regan Smith dominant and at the peak of their powers.
Superstar Dressel won five gold medals in Tokyo but stepped away from the sport in 2022 to tend to his mental health.
While the 27-year-old will not defend his 100m freestyle gold, he qualified for the 50m free and 100m butterfly and remains a significant contender in both.
Three-Peaty?
Another big-name veteran looking to turn back the clock is British breaststroke king Adam Peaty.
Like Dressel, he took time out to deal with depression but is back and in form as he seeks to become the first swimmer since Phelps to win the same race at three consecutive Olympics, in the 100m.
"Obviously now I do it to win, I want to be the best. I am the best," Peaty said.
But a crowded field is planning to dethrone him, led by China's Qin Haiyang who won all three breaststroke events at the 2023 world championships.
Qin is one of 23 Chinese swimmers implicated in a major doping scandal which only came to light this year and continues to hang over Paris.
They all tested positive for trimetazidine -- which can enhance performance -- ahead of the Tokyo Olympics, but were not sanctioned.
Instead, WADA accepted the explanation of Chinese authorities that the results were caused by food contamination.
An independent investigation cleared the world anti-doping agency of any wrongdoing, but not before an ugly war of words with the United States Anti-Doping Agency (USADA).
Eleven of the Chinese swimmers, including butterfly specialist Zhang Yufei, who won two golds in Japan, and another gold medallist in Wang Shun, are in France.
The hosts have their own star in Leon Marchand after his sizzling 4:02.50 in the 400m medley last year shattered Phelps' 15-year-old world record.
A five-time world champion, Marchand will also be heavy favourite in the 200m medley and 200m butterfly as he bids to cement a burgeoning reputation.
