To begin, in the mid-1930s, Hitler gave speeches and campaigned for gun control in the theory that with the most modern police and methods firearms were not needed, and he won, followed by a bloody confiscation. England also had gun control but felt secure because of the channel, their army, navy and air force. Hitler began his build-up, and later Neville Chamberlain appeased Hitler's demands for taking over another sovereign nation and WWII began.
When the allied armies were defeated by Hitler, including English army at Dunkirk, England realized an invasion was imminent and their people could not defend themselves. What to do: They asked America for help and the private citizens gave up firearms to help the English.
The United States also was facing aggression to their west. In Japan, thre was a council of war in the latter 1930s. The Japanese army wanted to assault the West Coast of the United States, but Admiral Yamamoto of the Japanese navy gave a speech, where he said he had been educated in the United States and had traveled extensively. During his travels he determined that every community and school had shooting clubs and competitions regularly with good marksmen. An attack on the West Coast would be foolish because after the shock wore off the Americans would fight and the Japanese army would be annihilated. The Japanese army knew of Admiral Yamamoto's plans for destroying the American fleet.