American Druidism
You'd assume that since Druids have so little cultural tradition in America, it wouldn't ever become popular here. But in the 1830s, when the Ancient Order of Druids established a lodge in New York City, they were largely received favorably. 17,000 Americans were members of Druid orders by 1839. Not bad for something that was primarily imported to the US in opposition to recognized fraternal organizations like the Freemasons.
The largest American Druid organization, the Ancient Order of Druids in America, was established in 1912, thus it wasn't just a passing trend. There were 35,000 American Druids by 1923. Even in 1963, new, influential major organizations like the Reformed Druids of North America were still being established. Even these new groupings sometimes have an absurd level of secrecy surrounding them. For instance, the Reformed Druids of North America openly acknowledge on their own website that they have little to no knowledge of the meaning or importance of their symbol beyond the fact that it was created in 1964.