The United States in the 1930s
The United States played no real role in European politics in the 1930s but her lack of interest in European affairs was an important factor in helping to bring about war. The USA was the most powerful country in the world – just as she is now. She was also a democracy and disliked Hitler's Germany.
Roosevelt, the President of the United States, could have threatened Hitler by promising to join Britain and France if they stood up to Hitler and found themselves at war. In 1935 Hitler announced that Germany would have a conscripted army of 500,000. This was a direct attack on the Treaty of Versailles, since Germany was allowed an army of no more than 100,000. Instead of confronting Hitler over conscription, Britain and France let it pass. It was clear to Hitler that neither of these democracies would stand up to him. If the United States, Britain, and France had joined together to enforce the treaty against Germany, Hitler may have backed down once and for all.
Neil Demarco: The era of the Second World War; Oxford University Press, 1993/2000, page 14