How did Churchill find new allies?

Germany is a terrible enemy, so new allies must have the strength equal to or more than Germany. At that time, only two countries with such conditions did not participate in the war, namely the Soviet Union and the United States. Churchill's attention turned to these two countries. He continued to maintain and strengthen private correspondence with President Roosevelt to discuss the most important Anglo-American relations and the world situation. Churchill still signed the letter "former naval personnel". He likes to play sweet words, even in wartime. He always gives various codes to various meetings and activities.