I'm questioning whether this attempt will be worth it, because 'Murica historical revisionism is rampant and facts that challenge these narratives are mostly seen as unwelcome and ignored...
But the preparations for the invasion were happening
before those events you cited. America was building towards a wartime economy since '39, and Roosevelt made open statements in '40 of his intentions to aid the Allies against the Axis.
A quote by Roosevelt's adviser John Galbraith is instructive: "When Pearl Harbor happened, we were desperate. ... We were all in agony. The mood of the American people was obvious – they were determined that the Japanese had to be punished. We could have been forced to concentrate all our efforts on the Pacific, unable from then on to give more than purely peripheral help to Britain. It was truly astounding when Hitler declared war on us three days later. I cannot tell you our feelings of triumph. It was a totally irrational thing for him to do, and I think it saved Europe."
So here we have an admission that the administration
wanted to be involved in the European front before the declaration of war giving America the pretext needed to sell its population on the idea, and moreover was concerned that Japan's attack might force them to focus
only on the Pacific.
If there weren't
other motives for the US to invade Germany at play, then the administration would not have had "feelings of triumph" upon Germany declaring war on America. That admitted reaction of the administration shows us that the declaration of war was only the pretext that gave the American administration the excuse to do what it already wanted to do in the first place.
So the question is,
why did America (by which I mean the American administration of that time)
want to invade Germany to begin with?? From
their own statements, they didn't want to invade
because of the declaration, they wanted American involvement
before being given a perfect pretext to do so by Hitler's foolish actions.
Thus your explanation for the motives of attacking Germany is wholly insufficient.
So why did the US want to be involved?? Well in 1940, Roosevelt spoke about how he wanted to help Britain against Germany in order to provide Britain with "arsenal of Democracy", about how aiding Britain was important for the sake of Democracy in the world.
That's right. "Freedom"
was the reason.
Although I half expect you prefer the 'Murica revisionist "We only wanted to do it 'cause we got attacked first" version of the tale, and ignore the vast public statements made about how we wanted to protect freedom and Democracy from Germany
before the declaration of war, or the attack on Pearl Harbor, were justifications that could be used.