Which of the following actions explains why britain officially entered world war ii?

the signing of the pact of steel between hitler and mussolini

the german invasion of the soviet union

the signing of german-soviet nonaggression pact

the german invasion of poland

"The German invasion of Poland" is one action among the following choices given in the question that explains why Britain officially entered World War II. The correct option among all the options that are given in the question is the fourth option or the last option. I hope that this is the answer that has helped you. 

Well, the debate lost a valuable, powerful speaker when he died.