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.

"... in the event of any action which clearly threatened Polish independence, and which the Polish Government accordingly considered it vital to resist with their national forces, His Majesty's Government would feel themselves bound at once to lend the Polish Government all support in their power. They have given the Polish Government an assurance to this effect.

I may add that the French Government have authorised me to make it plain that they stand in the same position in this matter as do His Majesty's Government."

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.