Which work is noted for paving the way to the invention of atonal compositions in the 20th century?

The work that is noted for paving the way to the invention of atonal compositions in the 20th century is Wagner's "Tristan und Isolde." This opera is significant because it uses chromatic harmony in ways that stretch traditional tonal boundaries. The harmonic progressions in "Tristan und Isolde," particularly the prolonged tension and unresolved harmonies found in the "Tristan chord," create a sense of ambiguity that challenges the listener's expectations of resolution. This characteristic tension between consonance and dissonance serves as a precursor to later developments in atonality.

Wagner's innovative harmonic language influenced many composers who followed, including Arnold Schoenberg, who would later explore atonal music. The techniques established by Wagner allowed future composers to break away from traditional tonality altogether, leading to the exploration of new musical expressions in the 20th century.