We have to attack them! Yes, we will get them when they won't expect it!
The United States provided close to 80% of Japan's oil reserves for a long time. Without the US's oil supply, Japan's navy would not be able to function.
December 7, 1941
During the summer of 1941, the United States cut off oil supply to Japan.
"Be on Alert."
This action angered Japan, firing their anger up to an attack on the US, aiming to destroy the US Pacific fleet.
A Day In Which
On December 7, 1941, Japan launched a surprise attack on the US. This shocked, angered, and startled the United States; they had severe casualties, destroyed battle ships, and ammunition.
However, the United States did have some idea that Japan was about to attack, they just didn't know where, but they did decode a message from Japan that said "be on alert," on the morning of the attack.
United States of America
Roosevelt told Congress to declare war on Japan, in which they did. The United States was now involved in a war because of December 7, 1941; "A day in which will live in infamy."