Here is an essay by one of the founders of Dada, Tristan Tzara (Sami Rosenstock a.k.a. Samuel Rosenstock; 1896 – 1963), who rambles on about the origins of the movement:
"Dadaism is a characteristic symptom of the disordered modern world...To the exiled intellectuals of Switzerland, humanity seemed to have gone insane - all order was crashing to destruction, all values were turned upside down - and, in accordance with this spirit, they began a set of wild practical jokes, elaborately silly meetings and fantastic manifestos which burlesqued, in their violence of the life around them."
In 1923, Tristan Tzara wrote an article about German Expressionism that can be read here.
