In 1939, Britain and France declared war on Germany after it invaded which country?

Poland is correct because Germany’s invasion on 1 September 1939 triggered British and French treaty commitments to defend Polish independence. They issued ultimatums and, when Hitler refused to withdraw, declared war on 3 September 1939—formally starting World War II in Europe. Context: Earlier Nazi moves—the Anschluss with Austria (1938) and the dismemberment of Czechoslovakia (1938–39)—had been met with appeasement, not war. But Poland was the red line. Germany struck with blitzkrieg tactics, while a secret clause of the Molotov–Ribbentrop Pact allowed the Soviet Union to invade eastern Poland two weeks later. Memory tip: “P for Poland, P for the Point of no return”—Poland marked the point when