<P> Even though Poland was overrun in about five weeks in the German Invasion of Poland beginning on 1 September 1939 and Soviet invasion beginning on 17 September 1939 , the Western Allies did nothing . War had been declared by each side , but no Western power committed to launching a significant land offensive , notwithstanding the terms of the Anglo - Polish and Franco - Polish military alliances which obliged the United Kingdom and France to assist Poland . </P>
why was the period between the winter of 1939 and the spring of 1940 called the phoney war
War had been declared by each side , but no Western power committed to launching a significant land offensive