I think of all the stupid things Hitler did, declaring war on the USA on December 11th 1941 was the worst. Not because of the obvious industrial might that the USA brought to the fight but because of the uncertainty it would have brought to the allied cause if he had not declared war.
I’m sure a large proportion of Americans would have wanted to concentrate on defeating Japan and Roosevelt would have had a tough job persuading his country to declare war on Germany when they were already committed to fighting in the Pacific. Hitler let him off the hook.
I'm sure the day that Hitler declared war on the US, Churchill lit the biggest cigar he could find , and opened a bottle of his very finest whiskey.
WWII had been raging for more than 2 years, and still the US had not declared war on Germany. This despite the exhortations of Churchill and the sinking of many ships and the loss of many American lives to the German U-Boats. It makes you wonder what it
would have taken for the US to declare war, and I'm really not sure that the attack on Pearl Harbour by Japanese forces would have been the catalyst.
When you go into a war, the last thing you need is to start it on two fronts directly opposite each other, with both requiring considerable numbers of men and munitions. Obviously after Germany declared war, the US had little choice but to intervene before the prospect of a Russian and/or British defeat which would have made the task immeasurably more difficult.
I can think of only 3 reasons why Hitler would choose to declare war at this time.
1.He thought that a declaration of war against Germany was certain, and he wanted to get in there first. And that as Japan was the most immediate threat, the US would concentrate against them, allowing the German navy to wage open warfare against US ships in the Atlantic.
2.He thought that by declaring war he would cement his alliance with Japan, and gain their aid in a pincer movement from the East. He also probably assumed that with the Japanese on a war footing and the US having significant damage to their Pacific fleet that it would take many years before they could effectively attack the Japanese.
3. Hitler was delusional that he really could defeat the US, when in reality he could barely mount an effective attack. If he'd waited until the V3 or V5, or had time to develop long range bombers or (perish the though) atom bombs then it was possible, though still incredibly risky. Whilst on the other hand the US could supply hundreds of thousands of troops and millions of tonnes of equipment to bolster the war in Europe.
If on the other hand Hitler had publicly decried the Japanese attack on Pearl Harbour and dismissed the pact with Japan (even if merely as a ruse) and perhaps offered military support to the US against the Japanese, it would have thrown things completely into disarray. I'm not sure that Roosevelt would have been able to sell his 'Europe First' plan to either his government or the people.