After the German invasion of Poland, the primary concern of both Roosevelt and his top military staff was on the war in Europe, but Japan also presented foreign policy challenges.
Relations with Japan had continually deteriorated since its invasion of Manchuria in 1931, and they had further worsened with Roosevelt's support of China.
With the war in Europe occupying the attention of the major colonial powers, Japanese leaders eyed vulnerable colonies such as the Dutch East Indies, French Indochina, and British Malaya.
After Roosevelt announced a $100 million loan (equivalent to $1.8 billion in 2019) to China in reaction to Japan's occupation of northern French Indochina, Japan signed the Tripartite Pact with Germany and Italy.
The pact bound each country to defend the others against attack, and Germany, Japan, and Italy became known as the Axis powers.
Overcoming those who favored invading the Soviet Union, the Japanese Army high command successfully advocated for the conquest of Southeast Asia to ensure continued access to raw materials.
In July 1941, after Japan occupied the remainder of French Indochina, Roosevelt cut off the sale of oil to Japan, depriving Japan of more than 95 percent of its oil supply.
He also placed the Philippine military under American command and reinstated General Douglas MacArthur into active duty to command U.S. forces in the Philippines.

The Japanese were incensed by the embargo and Japanese leaders became determined to attack the United States unless it lifted the embargo.
The Roosevelt administration was unwilling to reverse policy, and Secretary of State Hull blocked a potential summit between Roosevelt and Prime Minister Fumimaro Konoe.
After diplomatic efforts to end the embargo failed, the Privy Council of Japan authorized a strike against the United States.
The Japanese believed that the destruction of the United States Asiatic Fleet (stationed in the Philippines) and the United States Pacific Fleet (stationed at Pearl Harbor in Hawaii) was vital to the conquest of Southeast Asia.
On the morning of December 7, 1941, the Japanese struck the U.S. naval base at Pearl Harbor with a surprise attack, knocking out the main American battleship fleet and killing 2,403 American servicemen and civilians.
At the same time, separate Japanese task forces attacked Thailand, British Hong Kong, the Philippines, and other targets.
Roosevelt called for war in his famous "Infamy Speech" to Congress, in which he said: "Yesterday, December 7, 1941 — a date which will live in infamy — the United States of America was suddenly and deliberately attacked by naval and air forces of the Empire of Japan."
In a nearly unanimous vote, Congress declared war on Japan.
After the Japanese attack at Pearl Harbor, antiwar sentiment in the United States largely evaporated overnight.
On December 11, 1941, Hitler and Mussolini declared war on the United States, which responded in kind.

A majority of scholars have rejected the conspiracy theories that Roosevelt, or any other high government officials, knew in advance about the Japanese attack on Pearl Harbor.
The Japanese had kept their secrets closely guarded.
Senior American officials were aware that war was imminent, but they did not expect an attack on Pearl Harbor.
Roosevelt had expected that the Japanese would attack either the Dutch East Indies or Thailand.
After the German invasion of Poland, the primary concern of both Roosevelt and his top military staff was on the war in Europe, but Japan also presented foreign policy challenges. Relations with Japan had continually deteriorated since its invasion of Manchuria in 1931, and they had further worsened with Roosevelt's support of China. With the war in Europe occupying the attention of the major colonial powers, Japanese leaders eyed vulnerable colonies such as the Dutch East Indies, French Indochina, and British Malaya. After Roosevelt announced a $100 million loan (equivalent to $1.8 billion in 2019) to China in reaction to Japan's occupation of northern French Indochina, Japan signed the Tripartite Pact with Germany and Italy. The pact bound each country to defend the others against attack, and Germany, Japan, and Italy became known as the Axis powers. Overcoming those who favored invading the Soviet Union, the Japanese Army high command successfully advocated for the conquest of Southeast Asia to ensure continued access to raw materials. In July 1941, after Japan occupied the remainder of French Indochina, Roosevelt cut off the sale of oil to Japan, depriving Japan of more than 95 percent of its oil supply. He also placed the Philippine military under American command and reinstated General Douglas MacArthur into active duty to command U.S. forces in the Philippines.
The Japanese were incensed by the embargo and Japanese leaders became determined to attack the United States unless it lifted the embargo. The Roosevelt administration was unwilling to reverse policy, and Secretary of State Hull blocked a potential summit between Roosevelt and Prime Minister Fumimaro Konoe. After diplomatic efforts to end the embargo failed, the Privy Council of Japan authorized a strike against the United States. The Japanese believed that the destruction of the United States Asiatic Fleet (stationed in the Philippines) and the United States Pacific Fleet (stationed at Pearl Harbor in Hawaii) was vital to the conquest of Southeast Asia. On the morning of December 7, 1941, the Japanese struck the U.S. naval base at Pearl Harbor with a surprise attack, knocking out the main American battleship fleet and killing 2,403 American servicemen and civilians. At the same time, separate Japanese task forces attacked Thailand, British Hong Kong, the Philippines, and other targets. Roosevelt called for war in his famous "Infamy Speech" to Congress, in which he said: "Yesterday, December 7, 1941 — a date which will live in infamy — the United States of America was suddenly and deliberately attacked by naval and air forces of the Empire of Japan." In a nearly unanimous vote, Congress declared war on Japan. After the Japanese attack at Pearl Harbor, antiwar sentiment in the United States largely evaporated overnight. On December 11, 1941, Hitler and Mussolini declared war on the United States, which responded in kind.
A majority of scholars have rejected the conspiracy theories that Roosevelt, or any other high government officials, knew in advance about the Japanese attack on Pearl Harbor. The Japanese had kept their secrets closely guarded. Senior American officials were aware that war was imminent, but they did not expect an attack on Pearl Harbor. Roosevelt had expected that the Japanese would attack either the Dutch East Indies or Thailand.
Franklin D. Roosevelt