I agree there would eventually have been a US declaration of war, but when is the question.
I recently finished reading "The Pearl Harbor Myth", don't recall the author's name off the top. If one subscribes to the theory, Roosevelt had decided long before 12/7/41 that the European situation presented the greater threat and that it was essential to enter the war in the European theatre as a priority, and that preparations had to give recognition to that priority, even if it meant that a likely Pacific theatre was given less in way of resources. The trick was how to enter the war in Europe.
The analysis goes on that Roosevelt embarked on a deliberate programme of provocation on both fronts. If it ended up with Hitler declaring war, so much the better. If it ended up with Japan declaring war, whether attacking first or not, and thereby triggering a declaration of war by Hitler, then that was fine also.
The problem, so to speak, was that Hitler was ignoring the provocations, despite an essentially undeclared state of war. Absent a declaration of war, Roosevelt had to find sufficient cause in some incident or other that would not only justify a declaration of war against Hitler, but one that would also be of sufficient weight to rally popular opinion in the US fully behind him. He was still faced with considerable division of domestic opinion.
When sufficient cause might have arisen, or been perceived as such by Roosevelt, is a matter of speculation. One might imagine that chances are that with US naval forces increasingly engaged in convoy escort duties, a large unit would at some point have been sunk, whether deliberately or otherwise, providing "enough" cause.
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