At the outbreak of World War II Maria was on a lecture tour of India with her son Mario and as Italian citizens in a hostile country they were held under house arrest and internment respectively until the end of the war. Ada Montessori Pierson, Mario's wife to be, maintained AMI in a house in De Lairessestraat, Amsterdam during the war years until the return of Maria and Mario in 1947. Just prior to her return to Amsterdam in 1947 the house at 161 Koninginneweg was purchased for her as her home and as an office for AMI.