Roosevelt immediately blocked the sale of airplane fuel and scrap iron to Japan once Congress gave him the power to do so.
The Japanese was so upset that they made an alliance with Germany and Italy, becoming a member of the Axis.
Roosevelt began sending lend-lease aid to China. Japan invaded China in 1937 and controlled most of the Chinese coast. Roosevelt hoped that this aid would enable the Chinese to tie down the Japanese and prevent them from attacking somewhere but that failed.
American intelligence decoded Japanese communications and it was clear Japan was preparing war for the U.S. The U.S wasn't able to correctly interpret the information they received which left Pearl Harbor an open target.
December 7, 1941
Japan's surprise attack on Pearl Harbor was very devastating and caused real damage.
That Night Roosevelt met with his cabinet, telling them the country faced the most serious crisis since the Civil war. The next day, he asked Congress to declare war against the Japanese and thats what they did.