Question

What was the United States immediate reaction to the attack on Pearl Harbor?

Answer 1

The isolationist policy the US enforced during the first part of WW2 immediately changed after Japan attacked Pearl Harbor. They declared war on Japan and a few days later Italy and Germany declared war on the US

Answer 2

Here you go

The attack on Pearl Harbor immediately galvanized a divided nation into action. Public opinion had been moving towards support for entering the war during 1941, but considerable opposition remained until the attack. Overnight, Americans united against the Empire of Japan in response to calls to "Remember Pearl Harbor."