Were Japan to attack the Netherlands East Indies, a British declaration of war would have followed quickly.
Churchill would almost certainly have tried to elicit a U.S. declaration of war, too, from Roosevelt. That, however, is considerably less likely; isolationist sentiment remained considerable in Congress, and there would be little enthusiasm for defending a European colonial possession.
The problem, however, is what the U.S. *would* do instead. Roosevelt would sever diplomatic relations and do everything else short of war; and he would work feverishly to beef up U.S. forces and facilities in the Philippines, Guam, and Wake, put all U.S. forces on full war footing, and accelerate military acquisitions at full throttle. And when the U.S. *did* come in later in 1942, Japan will be facing a a U.S. sitting (in the P.I.) right astride its SLOCs to its oil in SE Asia, and very difficult and expensive to neutralize.
I think a DEI adventure by the Japanese in, let's say late 1940, brings a US declaration of war.
The U.S has enormous financial interests in the DEI oil industry and would never let these fall into the hands of Japan. Japan's possession of the NEI also destroys an oil embargo, cutting off any material way of hurting Japan and slowing their progress in China.