In short, since the defensive pact between Britain, France and Poland said nothing about the Soviet Union, the British and French refused to declare war against them. Read be
…low for more context. Prior to the German invasion of Poland on September 1, 1939, Poland, Britain and France made a defensive pact against the Germans, stating that if one of them was attacked, the other two would declare war against Germany. It is often stated that Britain and France responded to Germany's annexation of Czechoslovakia in March 1939 (a violation of the Munich agreement) by "guaranteeing the integrity of the Polish state". This is an incomplete explanation, as the agreement was for all three states to declare war against Germany (and it only pertained to Germany) if one of those three were attacked by Germany. Meanwhile, there was a political battle going on between Britain/France and Germany to get Soviet support in an eventual military conflict. Germany won this battle with a secret non-aggression pact that would partition Poland between the two states when they invaded it. This is what led to the German invasion of, and declaration of war on, Poland on September 1st. On September 3rd, the British and French, stunned by the boldness of Hitler (they had not expected him to actually attack), declared war on Germany, as per the agreement they had made with Poland. However, since the agreement did not specify military action, neither Britain nor France engaged Germany in any fighting until Germany invaded France the following year. This has been dubbed the "phony war", because of a lack of fighting by the British and French, but of course the war was ongoing in Poland, even after the capitulation of the Polish state and the exile of the Polish government to the British isles on September 28th, 1939. On September 17th, 1939 Poland was invaded on its eastern border by the Soviet Union. This attack, and the expectation of the attack itself, helped to split Polish defenses, and sped up the eventual defeat of the Polish military forces 11 days later. Since the defensive pact between Britain, France and Poland said nothing about the Soviet Union, the British and French refused to declare war against them, in keeping with their very technical and legalistic interpretation of the pact. Their hope was that Germany would stop with Poland, and not attack them next. One of the reasons cited for such a relatively easy defeat of Poland is that the Germans did not have to worry about the French troops on its Western border. German scouts were given the order to inform HQ if they saw so much as a single French soldier near the border, the response to which would have been a split of German forces to defend its western front while attacking Poland in the east. The scouts saw no one (possibly due to a combination of serendipity and an actual scarcity of French scouts near the border), so that virtually the entirety of Germany's forces were put into the attack on Poland, leaving Germany defenseless against a full western invasion from a superior number (at the time) of French forces for weeks. The lack of support from the Western "allies" and second frontal attack by the Soviets was too much for the Polish military, as was the single-front attack too much for France several months later. Only once Germany brought the Soviet Union and the United States (via their ally Japan's attack on Pearl Harbor) into the war against itself did it get defeated by a two-front war. (MORE)