From Wikipedia

Max Ernst and The Surrealist Revolution

After the war, filled with new ideas, Max Ernst, Jean Arp and social activist Alfred Grünwald formed the Cologne, Germany Dada group. In 1918 he married the art historian Luise Straus—a stormy relationship that would not last. The couple had a son who was born in 1920, the artist Jimmy Ernst. (Luise died in Auschwitz in 1944.) In 1919 Ernst visited Paul Klee and created paintings, block prints and collages, and experimented with mixed media.

In 1922, he joined fellow Dadaists André Breton, Gala, Tristan Tzara, and Paul Éluard at the artistic community of Montparnasse. Constantly experimenting, in 1925 he invented a graphic art technique called frottage (see Surrealist techniques), which uses pencil rubbings of objects as a source of images.

He also created another technique called 'grattage' in which paint is scraped across canvas to reveal the imprints of the objects placed beneath. He uses this technique in his famous painting 'Forest and Dove' (as shown at the Tate Modern).

The next year he collaborated with Joan Miró on designs for Sergei Diaghilev. With Miró's help, Ernst pioneered grattage in which he troweled pigment from his canvases. He also explored with the technique of decalcomania which involves pressing paint between two surfaces.

Ernst developed a fascination with birds that was prevalent in his work. His alter ego in paintings, which he called Loplop, was a bird. He suggested this alter-ego was an extension of himself stemming from an early confusion of birds and humans. He said that one night when he was young he woke up and found that his beloved bird had died, and a few minutes later his father announced that his sister was born. Loplop often appeared in collages of other artists' work, such as Loplop presents André Breton. Ernst drew a great deal of controversy with his 1926 painting The Virgin Chastises the infant Jesus before Three Witnesses: André Breton, Paul Éluard, and the Painter. In 1927 he married Marie-Berthe Aurenche, and it is thought his relationship with her may have inspired the erotic subject matter of The Kiss and other works of this year. In 1930, he appeared in the film L'Âge d'Or, directed by self-identifying Surrealist Luis Buñuel. Ernst began to make sculpture in 1934, and spent time with Alberto Giacometti. In 1938, the American heiress and artistic patron Peggy Guggenheim acquired a number of Max Ernst's works which she displayed in her new museum in London. Ernst and Peggy Guggenheim were also married to one another from 1942 to 1946.