The MASK traffic took place at a critical moment in Britain's history, with the country divided in its attitude towards the growth of Fascism in Europe, and the threat of Communism from Moscow. While there was an overt dimension to this political polarisation, with the CPGB enjoying widespread electoral support and a membership of over 50,000, and Sir Oswald Mosley's Blackshirts leading massive demonstrations, rallies and marches, there was also a very extensive covert side to both organisations, and they were the subject of surveillance and penetration by the Security Service.