Then, the soldiers left the war and spread flu all across the remote areas of the US.

Nah you are missing the point here. There was an already bad influenza strain, and millions of soldiers moving home on densely populated ships for weeks was a perfect environment for it to rapidly mutate and select for virulence and lethality. These soldiers making landfall then spread several strains of the virus, which led to vastly different mortality rates across the US. Much like we saw with Corona viruses rapidly mutating into different strains among the lockdowns.

Very true. Additioally, younger people had not been affected by any major outbreak. There were documented cases that older people, who had survived different previous outbreaks from were not hit as badly in the Spanish flu due to ore-existing natural immunity.

https://www.ncbi.nlm.nih.gov/books/NBK22148/table/a2000c209ttt00007/?report=objectonly

If interested this is a good read. Interesting bit:

"The New York City data also demonstrate that mortality among people aged 45 and older during the 1918–1919 pandemic influenza season was no worse than in surrounding years. For people under age 45, however, the 1918–1919 influenza season was very bad—people in this age group were far more likely to die of influenza than in previous years. Indeed, the age groups at highest absolute risk of dying during the 1918–1919 A(H1N1) pandemic were young children and young and middle-aged adults (Table 1-6).

These findings suggest that the early 1918 pandemic herald wave was spreading as early as February 1918, 6–7 months before the beginning of the explosive 1918–1919 pandemic. Relative to preceding influenza epidemic seasons, both the herald and pandemic waves caused proportionally more mortality in younger age groups but less mortality among those over 45 years of age, possibly as the result of recycling of an H1-like antigen from half a century earlier (Olson et al., 2004). "

https://www.ncbi.nlm.nih.gov/books/NBK22148/#a2000c209rrr00209