WWII did not start as a world war. When Germany attacked Poland the French and British declared war on Germany. But almost everyone else wanted to remain neutral. Germany then went on a ‘one at a time’ strategy. First Norway was attacked and conquered. Then Belgium and Netherlands (as they were in the way to get to France). Then Yugoslavia and Greece (although Greece was already at war with Italy). In each case the British wanted to assist these countries BEFORE the Germans invaded but in each case the country was afraid of angering Germany and thus be attacked. They each thought they could remain neutral. Each of them was wrong. By allowing Germany to attack countries one by one the Germans were able to move troops to the next position without fighting on multiple fronts. Their luck ran out with the attack on Russia but how much of a difference would it have made if all these countries had stuck together from the start of the war to stop Germany? And during all this time the United States was following the same official path of neutrality.

(Book – The Second World War – Winston S. Churchill)