Max, first off, Germany didn't need our money during the war, so financing is not an issue. Even though some American businesses were complicit in helping Germany rebuild itself during the 1930's, once they started invading other countries, they had an endless supply of loot and slave labor. The only issue was whether or not Americans sold them goods or did business, to profit off of them.
In a situation like UBC, if a company, even a large bank like that, was caught doing business with the enemy, even if members of their board of directors were U.S. Senators, their assets were seized. With draconian consequences like that, not too many people tried it.
As much as I dislike Bush, the notion that Prescott Bush was actively aiding the enemy, while his son was actively fighting against them, is laughable. George H. W. Bush was a TBM pilot, fighting against the Axis. Who are you going to back, the Axis or your son? I mean, the Bushes may be ultra rich, robber barons, but they don't eat their young.