Hi Molly,

This NARA page has detailed info on U.S. passport history and regulations.
https://www.archives.gov/research/passport

From the above site:

• Passports were recommended, but not required, by President Woodrow Wilson's Executive Order 2285 of December 15, 1915, which stated that all persons leaving the U.S. should have passports.
   
• Passports were required from May 22, 1918 (40 Stat. 559), until the formal termination of World War I in 1921 by treaties.
   
• Passports have been required since the passage of the act of June 21, 1941 (55 Stat. 252) and subsequent legislation.

Interestingly, there were brief periods in time when someone who was not yet a U.S. citizen, but had declared their intent, could obtain a U.S. passport.

Hope this info helps,

Sherri Bobish
Princeton, NJ