It is not difficult to find that the "US conditioning other countries in the 1970's" actually started in 1886 at the https://en.wikipedia.org/wiki/Berne_Convention when 10 european countries agreed on legal principles to protect original works. Among these 10, France, Germany, Italy, France and UK, so in practice the whole Western Europe. US didn't join until 1989.

The original treaty, if I am not misunderstanding here: https://www.wipo.int/en/web/treaties/ip/berne/summary_berne includes a "dead + 50 recomended" protection since the 1908 revision, before that it was up to each country laws, and in 1948 it changed to "dead + 50 minimum mandatory". In 1993 it was raised to "dead + 70" in the UE, to be followed by the US with the same extension in 1988 in Sonny Bono Act https://en.wikipedia.org/wiki/Copyright_Term_Extension_Act.