It was felt that Germany posed a greater threat than Japan.
Germany was the primary threat to Great Britain, so Churchill did not really need persuading about that. It was also feared that Germany was the greater threat to the Soviet Union. President Roosevelt was under pressure in the United States to focus on Japan because of Japan's attack on Pearl Harbor, but Germany's declaration of war against the USA (on Dec. 11, four days after the Pearl Harbor attack) made the Europe First strategy agreeable for the USA. The Europe First policy was decided at the Arcadia Conference held in Washington, DC, Dec. 22, 1941 to Jan. 14, 1942.