Declaring war on the US. This guaranteed that we would in turn declare war on them, which otherwise may not have happened, at least for a few more years. Germany hadn't directly attacked us, and so our refusal to declare war on them before Pearl Harbor likely wouldn't have changed.
Best bet for Germany would've been to allow Japan to take on the US to distract us while they continue to usurp Europe.
Interesting story. My Dad was a long time employee of the New York Central and Penn Central Railroads out of Cleveland. A buddy of his had emigrated from Germany around 1933 or 34 and was looking for work at the railroad. They got to talking about his family back home. His friend, whose name now escapes me, would get letters from his relatives urging him to return to Germany. The Fuhrer, they assured him, was the most brilliant man to ever walk the earth. They didn't think Germany would ever fight America because, they claimed, there were so many Germans and people of German descent, it would not be logical. Still, they told him America -- if there would be war -- would be quickly dispatched. Americans were lazy cake and pie eaters, they said. Undisciplined and incapable of sacrifice. And a cripple for a President, to boot!
He was tempted to buy into that line of thinking because, after all, what passed for poverty in depression-era Cleveland was nothing like what he knew of post-Great War Germany. And still the Americans complained! He thought perhaps his relations were right. Germany could defeat America.
Until he applied for a job at a warehouse in Cleveland. I don't remember the company, but he told my father the warehouse covered several soccer fields. And it contained nothing but pallets, ten feet high, of Coca Cola. From stem to stern. At the height of the depression.
At first, he thought, "The family is right. These people value this liquid candy so highly, they must be of little consequence." But the more he mulled it over, he came to another conclusion. He said to my Dad, "Mein Gott! What fools we were. To ever think Germany could defeat America was the height of arrogance. If America, in a time of economic depression, could produce that much Coca Cola in just one city -- imagine how many tanks and planes and rifles she could produce if she had to."
That story always stayed with me. I don't know if we're still that same kind of country. I hope so. But one thing I am sure of...there was never any chance of any country defeating us in 1941. No matter how unprepared we were, initially, for war.