 On 6 April, Poland and Britain agreed to formalise the guarantee as a military alliance, pending negotiations.On 28 April, Hitler denounced the 1934 German–Polish Non-Aggression Pact and the 1935 Anglo–German Naval Agreement.In mid-March 1939, attempting to contain Hitler's expansionism, the Soviet Union, Britain and France started to trade a flurry of suggestions and counterplans regarding a potential political and military agreement.