The correct answer is D) the German invasion of Poland.
The other options of the question were A) the signing of the Pact of Steel between Hitler and Mussolini. B) the German invasion of the Soviet Union. C) the signing of German-Soviet Nonaggression Pact
The action that explains why Britain officially entered World War II was the German invasion of Poland.
British Prime Minister Chamberlain made everything he had on his hands to maintain a foreign policy of appeasement to give Hitler what he wanted and avoid another world war. Europe suffered so much pain and destruction during World War 1, and Great Britain tried to avoid war at all costs. Every changed in 1939 when Adolph Hitler ordered the German troops to invade Poland. World War II had begun.
