Lend-Lease Act permitted U.S. to lend or lease arms to any country "vital to the defense of the United States." Usually this ended up being Allied nations. The Neutrality Act of 1939 allowed warring nations to buy weapons from the United States if they paid cash and transported arms on their own ships. Most of these weapons ended up with the British and their allies across the Atlantic.
Japan was also motivated strategically by ideas of creating an Asian co-prosperity sphere—“Asia for Asians”—in which Japan would take over the Asian colonial holdings of Europe and the United States.
The Japanese government decided to attack Pearl Harbor after the United States cut off US oil exports to Japan in the summer of 1941. Japan relied on the United States for eighty percent of its oil, and without US oil supplies its navy would be unable to function. In attacking Pearl Harbor the Japanese hoped to cripple or destroy the US Pacific fleet so that the Japanese navy would have free reign in the Pacific.4
On the morning of December 7, 1941, Japan attacked the US naval base at Pearl Harbor, Hawaii
The surprise attack by some 350 Japanese aircraft sunk/badly damaged eighteen US naval bases, including eight battleships, destroyed or damaged 300 US aircraft, and killed 2,403 men.
After the attack, the US declares war on Japan while Germany declares war on the US on December 8, 1941