What did Germany do to threaten US neutrality?

Answer 1

Not much. The President of the United States at the time was already hostile towards Germany before the war started for whatever reasons, but put up a charade to the American public of wanting to remain neutral. In mid 1941 US ships were ordered to attack German U-boats on sight, even though president Rooseveldt was still promising the people of America that the US would remain neutral.

The delicate neutrality was eventually broken by the Japanese attack on the United States Navy at Pearl Harbour, after a great deal of American provocation. War against Japan was the covert strategy of President Rooseveldt for the US to enter into war against Germany with mass public support.

Whether a victorious Germany would have been an actual threat to the US can be debated until judgement day without resolve.

German U-boats sank US ships (WWII), this was enough provocation on its own. Sinking the Lucitania(sp?) got us into WWI.