1) The attack began American involvement in WW2, until them the government had been supporting the Allies but the people wanted to remain neutral and not get involved in what they considered a “European War”. That changed when Germany declared war on the US several days later.

2) United the American people firmly on the side of the Allies. (no division like we see now-a-days)

3) Rendered the American battle fleet crippled for the beginning of the war forcing naval admirals to use carriers. Up until this point it was view the carrier would support the battleships. By the middle of 1942 the carrier would be established as the dominant weapon of the sea.

Just a few reasons to get you started.

The attack on Pearl Harbor was the straw that broke the camels back and was the trigger that pulled us in to the European and pacific theaters.

~Isoroku Yamamoto