But would Roosevelt have managed to get Congress and the public to support a war with Japan, on the basis that they are defending imperialism and colonialism. I hardly think a rallying cry of "Save the dutch and english colonies" would have the same effect as "Remember Pearl Harbor". I highly doubt it. Even if the US had declared war it would be a USA "woken from its slumber" and full of a terrible rage, they would instead fight the war halfheartedly. If the Japanese manage to score a few early victories, public opinion would turn massively against the war and isolationists would be screaming why are we defending colonialism, and in short order a peace would have been sought with Japan.
The US was virtually in a state of war with Germany throughout the majority of 1941; it's very likely that the US and Germany would have ended up at war soon enough. It isn't difficult for them to finally move in sometime 1942 for the same reason they declared during WW1. That, and they were going to support
Britain against the Germans. If Britain ends up in a war against Japan, it doesn't become a war to save the colonies. It's a war to protect an ally and its capability to wage war against a foe that both nations agree needs to fall.
And don't discount the China lobby, either. Whatever the colonialists do in their own colonies is hard to compare with the Japanese actions in China (Nanking et al), combined with the Japanese encirclement of the Philippines, combined with them strangling and cutting off contact with China (and threatening Australia and New Zealand) would be too much for risk. And it's not like the Japanese are liberating those regions; they are subjecting them to their own brand of colonialism: the colonialism argument doesn't exactly lend it much credence.
And, if the Japanese pass by the US, that gives them more time to fortify the Philippines and train/expand the army even more, while the small island bases also become much tougher nuts to crack. Combine that with the Two Ocean navy finally coming into play the later it drags out, the US will have a fleet that will be much larger and much newer than anything the Japanese have. And, so long as the US has the Philippines, that's a dagger pointed at the heart of the Japanese Empire. The Japanese can't live with the Sword of Damocles hovering above its head.
The mistake here is the same one the Japanese took. They look at the US and see them as weak and unwilling to fight. Even discounting the surprise attack, the US lost quite a lot at the start of the war. From the Philippines to Wake island, they were driven from their Pacific possessions. That would have been the perfect time to bow out. The core of the fleet was gutted and they had lost all of their possessions, but they didn't. In this situation, the US still maintains their entire fleet, and the Japanese are continuing to be even more aggressive and expansionist.
They've occupied Korea, China, French Indochina, and now the Dutch East Indies and the British East Indies. They are certainly not fighting a war to liberate them; they are conquering foreign territory to try to get relief from embargoes placed upon them because of prior heinous actions! If anything, the trend was rising
for intervention throughout the proceeding year, with clear majorities in favor for it.
It also doesn't help that Japan's actual potential for warmaking (3% of world total, 1937) was roughly equivalent to
Italy (2.5% of world total, 1937). They had just superheated their own economy in order to keep up the construction that they
did maintain. The Japanese were quite aware of the economic disparity, which was the reason behind the surprise attack in the first place. To continue to let the US build up was untenable; any further aggression by them would result in such plans being accelerated, as they had previously.