Britain and France declared war on Germany on September 3, 1939, primarily in response to Germany's invasion of Poland on September 1, 1939. The invasion of Poland was a clear violation of international agreements and treaties, including the Treaty of Versailles, which ended World War I and aimed to maintain peace in Europe.
Both Britain and France had pledged to support Poland in the event of aggression against it, as part of their commitment to collective security. Following the invasion, they issued ultimatums to Germany, demanding its withdrawal from Poland. When Germany failed to comply, Britain and France declared war.
The declaration of war was motivated by a desire to contain German expansionism, protect Poland, and prevent the broader consequences of unchecked aggression in Europe, which ultimately contributed to the outbreak of World War II. This conflict was seen as a response to the aggressive policies of Adolf Hitler and the Nazi regime, and the subsequent efforts by Britain and France to uphold European stability and order.