I believe the answer is c.1920. However, I could be wrong.
U.S. treaties and international agreements currently in force (i.e., excluding those, some of which are included on this page, that are no longer in force, and that are signed but not ratified or otherwise have not yet entered into force), divided between (1) bilateral treaties organized by state and then by topic, and (2) multilateral treaties organized by topic, see the annual State Department publication.
I have an incomplete list but I hope it helps.
Contents
1 Pre-Revolutionary War treaties
2 U.S. international treaties
2.1 1776–1799
2.2 1800–1849
2.3 1850–1899
2.4 1900–1949
2.5 1950–1999
2.6 2000–current
3 U.S.–Native American treaties
3.1 1778–1799
3.2 1800–1809
3.3 1810–1819
3.4 1820–1829
3.5 1830–1839
3.6 1840–1849
3.7 1850–1859
3.8 1860–1869
3.9 1870–1879
3.10 1880–present
Hope this helps! ^^
