I think Japan had already decided to attack us before they signed the Tripartite Pact.
Actually until Admiral Yamamoto was promoted to head of the Japanese Naval forces, the Japanese hadn’t planned attacking at Pearl Harbor because up until 1940 most of the U.S. Pacific Fleet was at San Diego. The Japanese did figure that when they invaded the Philippines, the U.S. fleet would sail with relief forces and along the way Japanese submarines, light forces and planes would attrition this force until a Jutland style battle would decided it. The battleships of the Yamato class were built as Japan figured the U.S. wouldn’t build any battleship larger than could fit through the Panama Canal. Admiral Yamamoto changed this strategy as he had a pretty good idea that a long war would end in defeat for Japan and he wanted to strike first (the Essex class carriers were already being laid down). The new movie Midway states it was our greatest Intelligence Failure, that statement was wrong, it was our greatest Command and Control failure, including the command of U.S. forces in Hawaii (between the Army and Navy, there was some 300 U.S. planes based there, but only 3 PBY planes were out doing general recon’s and no Combat Air Patrol above the base). FDR was pushing Japan to attack because he knew that the U.S. needed to get into the war, but due to complacency and arrogance on our part, very few thought that Hawaii would be subject to a major attack. As for our carriers, one was supposed to be in harbor that day (the U.S.S. Enterprise), but a storm delayed its return to Pearl Harbor (talk about a lucky break!). During the Japanese attack, Dauntless dive bombers from the Enterprise flew in and six were shot down from our own Anti-aircraft no less.
Not the case. The tripartite pact was signed in Sept. 1940. Planning for the strike South and at Pearl Harbor did not start until early Spring 1941.