The Decline of Romanticism

20th century modernism was largely a reaction to heavy-handed German late romanticism, 
especially Richard Wagner's long operas about death, redemption, heroism, etc.  

Some late 19th century composers who influenced the early modernists:

Russian composer Modest Mussorgsky created new methods of creating melody.  This influenced Claude Debussy when he heard Mussorsky's music at the 1889 World's Fair in Paris.

Debussy's impressionism was influenced by late-period Franz Liszt compositions.

New interest in Asian music influenced Debussy and other.

Russian Igor Stravinsky's new approach to rhythm - polyrhythms - (Africans had been doing this for centuries)

Frenchman  Gabriel Fauré lighter style of music

Atonality was influenced by ? (Mahler)?