From 1918-40 all of his plays were produced at his own Paris venue, Theatre du Gymnase and later the Theatre des Ambassadeurs. His anti-Nazi play "Elvire" ran at the latter until shortly before Paris fell to the Germans on 6/14/40.
Fled to the US shortly before France fell to the Nazis in 1940. While there he spoke and wrote on behalf of Charles de Gaulle and served as president of the French-American Club. Bernstein returned to France in 1946.
In the First World War he served with the British artillery during the Battle of Flanders and later with the French air corps in Macedonia.
As a playwright, he was not known for taking criticism well. Bernstein was said to have participated in twelve duels with other authors and critics, none resulting in serious injury to either party.