[Romulus]

There was nothing shady about their dealings with Nazi Germany.

So I suppose it was all a crazy mistake when Union Banking Corporation, in which Bush was a director, was seized by the U.S. government on Oct. 20, 1942, under the Trading with the Enemy Act.

[Little Ray]

Probably.
UBC belonged to Fritz Thyssen, and Prescott Bush managed it for him. Before UBC was seized, Thyssen had a falling out with Hitler (over antisemitism of all things!). When UBC was seized, Thyssen was in a Nazi concentration camp and all his European property had been confiscated by the Third Reich.
Likely as not, Mr. Bush was simply being a good steward of the properties he was mangaging. He may have even kept the Thyssen's oversea's assets out of Nazi hands. This thing is apparently a LOT more complicated than it looks...

[cartan]

The Trading With the Enemy Act was signed only after the Pearl Harbor attack. Bush wasn't arrested or anything, either; instead, he got $1,500,000 in reimbursement for his one share of the Union Banking Corporation. Also, note that the UBC was formed, in 1924, by Thyssen, not the National Socialists.