As I said recently, modernism began its assault on the Truth in the late 19th century. St Pius X, with Lamentabili sane exitu; Syllabus Condemning the Errors of the Modernists; Pascendi Dominici Gregis, On the Doctrines of the Modernists; and Sacrorum antistitum (Latin), containing the Oath Against Modernism (English), thought he had defeated them. Unfortunately for us, over a century later, all it did was drive the heretics underground. There, they bided their time until Vatican II when they came roaring back to wreak havoc on the Church of Christ.
From One Peter Five