It was, I believe, essentially a condemnation of the “liberal” American ideals of religious liberty, separation of chuch and state, etc., which came under the overarching condemnation of modernism. The Church then equated the American political system with the ideals of the French Revolution, which it fought tooth and nail. Generally, it perceived them both as putting Man above God. I think there was also an admonishment of the perceived “liberalism” of American priests and bishops. (Some things never change!)
I’m sure there will be a lot of disagreement on this point, but I think this has been basically abrogated by Vatican II, which renounced the temporal authority of the Church and accepted religious liberty. I think most of the Popes since the 1940s would probably say that “Americanism” as then defined is not heretical, but I’m kind of speculating based on the languages the Popes use today. Hope this helps!
Also interesting to note that Fr. Isaac Thomas Hecker, whose views as set forth in his posthumous biography sparked the original controversy, was put forth for sainthood by Cardinal Egan last year.
According to Wikipedia (I know a lot of people distrust it, but I read the source documents too!) the controversy had more to do with political pressures from the French bishops, and that the whole affair passed without the laity ever knowing much about it. That wouldn’t happen today, you can bet!
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