Why didn’t the Allies Attack Germany in 1939?

On September 1, 1939, Germany invades Poland, setting off the Second World War. Two days later, Britain and France declare war on Germany. As the German army races towards Warsaw, many German generals are worried the French might simply walk into western Germany, and there’s not much the Wehrmacht can do about if they do. But instead of a powerful Allied counteroffensive, the French and British mostly sit back and wait during the so-called Phoney War – so why didn’t the Allies attack Germany in 1939?