Why did japan's attack on pearl harbor lead the US to declare war?

Answer 1

The Japanese attack was an overt & intentional act of war against the United States that required the United States to respond. The unprovoked Japanese attacks at Hawaii, Wake Island, Midway Island, Guam, and the Philippines killed thousands of Americans. The US Declaration of War was requested by the President and formally approved by the US Congress the next day.

Answer 2

so they can bomb the U.S. battle ships and U.S. carriers, so the U.S. won't be able to inter fear with japans plans in conquering all of south East Asia.