The Tripartite Pact only committed its members to an alliance against other nations that initiated war against a Pact member. Nothing required Germany to declare war on the US after Pearl Harbor. You’re saying that Roosevelt gambled the future of the nation on a war the course of which he couldn’t ascertain. And as for the US knowing about the impending attack, we had reason to believe that Japan was going to strike somewhere in the near future. But the expectations were for an attack in the Philippines or SE Asia. Nobody seriously considered a carrier strike against Hawaii, largely due to the prevailing mindset in the US military which considered Japan to be lacking in the capability or the guts for such a bold, complex action. There was no information received by the US which clearly pointed to an attack on Pearl Harbor on 12/7/41. Japanese operation security for the attack was airtight, anyone who didn’t need to know was not informed.
I don't believe that for a second...the Pacific Ocean was alive with international shipping and air traffic...to think that the Japanese fleet could have crossed it without being detected is nonsense....Roosevelt wanted into that war and many pacificists in this country did not. Many of our most important allies were already involved and he knew that they were incapable of carrying the load....Our industrial power was, by far, the most formidable in the world and soon turned into a war machine.