What was the main idea of the monroe doctrine?

The main idea of the Monroe Doctrine was the declaration of American supremacy in the affairs of North and South America. Through this declaration the United States attempted to prevent any European or external involvement in the affairs of the Americas. 

Britain and its empire stood alone against hitler and mussolini. in june 1941, hitler broke the non-aggression agreement with stalin and germany invaded the soviet union. in december, japan attacked the us and britain. the main lines of world war ii had formed.