Short answers, pulled from the story.
The word avant-garde originated in the French military tradition of the 19th century to designate the advance guard of an army tasked with scouting terrain ahead of the main force. By the mid-1800s, French political reformists adopted this terminology to describe left-wing agitators who sought radical social change. The term migrated from the battlefield to the salon when artists began to view their experimental work as a form of reconnaissance.
The year 1915 marked the birth of Dada, a movement that originated in Zurich during the height of World War I. Dada artists like Hugo Ball and Tristan Tzara created works that were intentionally nonsensical to shock the public and dismantle cultural institutions. Marcel Duchamp's 1917 submission of a urinal titled Fountain to an exhibition of the Society of Independent Artists in New York stands as the most iconic moment of this era.
Figures like Ezra Pound, Wyndham Lewis, and the Italian futurist F.T. Marinetti found themselves aligning with right-wing movements including Nazism and Fascism in the early 20th century. Marinetti, the founder of Italian Futurism, openly praised the violence and aggression of the fascist state. The Soviet avant-garde initially embraced the Russian Revolution as a chance to create a new art for the new society before facing increasing censorship and persecution.
The academic analysis of the avant-garde began in earnest with Renato Poggioli's 1962 work The Theory of the Avant-Garde. Poggioli surveyed the historical, social, psychological, and philosophical aspects of artistic vanguardism and identified that avant-garde artists shared values and ideals with contemporary bohemians. He also noted the inherent contradiction in their position regarding their relationship to the establishment.
The 1960s saw a wave of free and avant-garde music in the jazz genre embodied by artists such as Ornette Coleman, Sun Ra, and Albert Ayler. The movement included figures like Edgard Varèse, Alban Berg, George Antheil, Harry Partch, Iannis Xenakis, Morton Feldman, Karlheinz Stockhausen, Pauline Oliveros, Philip Glass, Meredith Monk, Laurie Anderson, and Diamanda Galás. The 1960s and 1970s saw the emergence of free jazz and post-punk which rejected traditional rock sensibilities in favor of an avant-garde aesthetic.
Theater and performance art became the primary vehicle for avant-garde expression in the 20th century with movements like Fluxus, Happenings, and Neo-Dada challenging the traditional boundaries of the stage. The Fluxus movement founded by George Maciunas in the 1960s emphasized the process of creation over the final product. Happenings were events that took place in non-theatrical spaces and involved the participation of the audience to blur the lines between art and life.