Anglo - Polish military alliance The Anglo - Polish military alliance refers to the alliance between the United Kingdom and the Polish Second Republic formalised by the Anglo - Polish Agreement in 1939 and subsequent addenda of 1940 and 1944 , for mutual assistance in case of military invasion by Germany British Guarantee to Poland . On March 31 , 1939 , in response to Nazi Germany ' s defiance of the Munich Agreement and occupation of Czechoslovakia , the United Kingdom pledged the support of itself and France to guarantee Polish independence . On April 6 , during a visit to London by the Polish foreign minister , it was agreed to formalize the guarantee as an Anglo - Polish military alliance , pending negotiations . This guarantee was extended on April 13 to Greece and Romania following Italy ' s invasion of Albania . Polish - British Common Defence Pact . On August 25 , two days after the Nazi - Soviet Pact , the Agreement of Mutual Assistance between the United Kingdom and Poland was signed . The agreement contained promises of mutual military assistance between the nations in the event either was attacked by some " European country " . The United Kingdom , sensing a dangerous trend of German expansionism , sought to prevent German aggression by this show of solidarity . In a secret protocol of the pact , the United Kingdom offered assistance in the case of an attack on Poland specifically by Germany , while in the case of attack by other countries the parties were required to " consult together on measures to be taken in common " . Both the United Kingdom and Poland were bound not to enter agreements with any other third countries which were a threat to the other . Because of the pact ' s signing , Hitler postponed his planned invasion of Poland from August 26 until September 1 . Analysis . At the time Adolf Hitler was demanding the cession of the port of Danzig , an extraterritorial highway ( the Reichsautobahn Berlin - Königsberg ) across the Polish Corridor , and special privileges for the German minority within Poland . By the terms of the military alliance , each party ( i . e . Poland and Britain ) was free to decide whether to oppose with force any territorial encroachment , as the pact did not include any statement of either party ' s commitment to the defence of the other party ' s territorial integrity . The Pact did contain provisions regarding " indirect threats " and attempts to undermine either party ' s independence by means of " economic penetration " , a clear reference to the peculiar status of Danzig . Fearing all - out German invasion no matter what , Poland rejected the German demands . The British and French governments had plans other than fulfilling their treaties with Poland . On May 4 , a meeting was held in Paris at which it was decided that " the fate of Poland depends on the final outcome of the war , which will depend on our ability to defeat Germany rather than to aid Poland at the beginning . " Poland ' s government was not notified of this decision , and the Polish - British talks in London were continued . Also in May 1939 , Poland signed a secret protocol to the 1921 Franco - Polish Military Alliance , but it was not ratified by the French until September 4 . On September 17 the Soviet Union invaded Poland through the eastern Polish border . According to the Polish - British Common Defence Pact , the United Kingdom should give Poland “ all the support and assistance in its power ” if Poland was " engaged in hostilities with a European Power in consequence of aggression by the latter " . The Polish ambassador in London , Raczyński , contacted the British Foreign Office pointing out that clause 1 ( b ) of the agreement which concerned an " aggression by a European power " on Poland , should apply to the Soviet invasion . The Foreign Secretary Lord Halifax responded that the obligation of British Government towards Poland arising out of the Anglo - Polish Agreement , was restricted to Germany , according to the first clause of the secret protocol .