Japan had decided they needed more power. So, the Japanese started to colonize and take over other Pacific Asian locations. They took over the places that could maximize their resources. Other countries noticed.
So many DEAD!!!
America was Japan's best provider of resources. So to stop Japan's expedition, America planned to cut off their resource output if Japan didn't withdraw their troops. Japan had to pick between their best provider or future possibilities of power.
FDR: "Date that will live in infamy
December 7th, 1941 a date that will live in infamy
Japan decided to continue their conquest and for them to gain control of the Pacific, they couldn't continue to fight America. So to get America out of the way, Japan attacked Hawaii, more specifically Pearl Harbor. They thought if they could cripple the American Navy then they could continue their conquest.
We are ramping up weapon production for the war
There were many effects that happened immediately. Adding to the mounding causalities, over 2000 dead, many ships were damaged including many that were sunk. This caused the American to rise up and support the war. Soon after FDR gave a speech, a quite famous one actually
On December 8, 1941 FDR gave a speech where he addressed the state of war America was in and the attack on Pearl Harbor. He coined the term of "Date that will live in infamy". Americans rallied around this and Congress declared war on Japan.
This attack had many long-lasting effects. First, and arguable the most important, America, the most powerful country, was ticked off. They immediately started mega recruiting and drafting. They lunged out of the the Great Depression and focused all industry on the War. This also caused the eventual nuclear bomb drops on Japanese cities.