Final answer:
Britain and France declared war on Germany two days after the German invasion of Poland, on September 3, 1939.
Step-by-step explanation:
The German Wehrmacht invaded Poland on September 1, 1939, which triggered the onset of World War II. After the invasion, it took only two days for Britain and France to react and formally declare war on Germany. This declaration occurred on September 3, 1939, when both countries began mobilizing their armies, though no significant military assistance was provided to Poland at that time. This period also marked the end of the policy of appeasement that was initially aimed at preventing the wrath of Nazi Germany.