Dada had begun in Zurich during World War I, and became an international phenomenon. Dada artists met and reformed groups of like-minded artists in Paris, Berlin, Cologne, and New York City. In Germany, Richard Huelsenbeck established the Berlin group, whose members included Jean Arp, John Heartfield, Wieland Hertzfelde, Johannes Baader, Raoul Hausmann, George Grosz and Hannah Höch. Machines, technology, and a strong Cubism element were features of their work. Jean Arp and Max Ernst formed a Cologne Dada group, and held a Dada Exhibition there that included a work by Ernst that had an axe "placed there for the convenience of anyone who wanted to attack the work". Kurt Schwitters established his own solitary one-man Dada "group" in Hanover, where he filled two stories of a house (the "Merzbau") with sculptures cobbled together with found objects and ephemera, each room dedicated to a notable artist friend of Schwitter's. The house was destroyed by Allied bombs in 1943.