This is an interesting site that shows America’s shift in sentiment towards entering WWII over time.
The war began in 1939 with Germany invading Poland, followed by invasions into Netherlands, Belgium, and France.
At this point both Britain and France had already declared war on Germany, but many Americans were in the “isolationist” camp, perhaps a result of the depression of the previous decade and the lack of an immediate threat to national security.
By the time of Pearl Harbor in 1941, 91% of the country was in favor of WWII, nearly a complete reversal from the year before.