Starfleet is a direct descendant of United Earth Space Force which is indirectly a descendant of previous militarized space programs like NASA/Roskosmos. I'm sure the Vulcans gave appropriate side eye when told their science officers got military ranks.

Sorry, UE Space Probe Agency, not UESF (and that name dates from Star Trek's first season in the 60s) memory-alpha.fandom.com/wiki/United_...

United Earth Space Probe Agency

The United Earth Space Probe Agency, commonly abbreviated UESPA, was a space exploration agency with its origins dating back to the era of Earth's post-atomic horror. This agency was administered by U...