The problem is your incomprehension of what I actually wrote and of what a Japanese offensive in the Pacific would actually entail. Let's tackle the second point first.
Japan is going on the offensive in the Pacific in order to grab the DEI and other resource producing regions in southeast Asia. In order to safeguard her LOCs between the Home Islands and the regions she's targeting, Japan must also attack the US forces based in the Philippine islands. That brings the US into the war whether the Pearl Harbor raid occurs or not.
Japan must also target
British possessions in Asia like Hong Kong, Malaysia, and Singapore and
British protectorates like Brunei for the same reason she needs to attack the Philippines: the lines of communication between the DEI and the Home Islands must be made secure.
Given her strategic goals, any Japanese offensive in the Pacific will embroil that nation in a war with the US
and Britain. That's the point I was making and the point you have failed to understand.
Once the US is involved in a war against Japan
with Britain as an ally, the US will begin shipping materials to the British islands as part of the war effort against Japan. Germany cannot let those materials through because, as I pointed out, there is no Cosmic Accountant making sure what the US ships to Britain will only be used in the war against Japan.
Germany cannot differentiate between US ships carrying supplies "only" for the war with Japan and there's no actual difference to determine anyway. As I explained, the iron ore, copper, and Spam provided by the US to supply and feed a munition worker won't only be used and eaten when that worker is producing bombs to be dropped on Japan. Germany will have to attack US ships carrying US supplies to a US ally because any supplies will help Britain with her fight with Germany. Sinking US ships carrying US supplies to a US ally will then provide the US with the
casus belli FDR has been looking for all along.
So, what's the difference between the US-sourced, British-bound cargoes being sunk by U-boats in the Atlantic on December 6th and the US-sourced, British-bound cargoes being sunk by U-boats in the Atlantic on December 8th? The difference is that on December 6th the US and Britain
are not de jure allies in a war against Japan and on December 8th the US and Britain
are de jure allies in a war against Japan.
Before December 7th, Britain is either purchasing supplies or leasing them and then arranging to have those supplies transported across the Atlantic. The US is aiding Britain as much as she can while still maintaining some semblance of neutrality. Along with providing or lending materials, RN warships are being overhauled and even constructed in US yards. The US is also "defending" the Western Hemisphere against Axis aggression by aggressively patrolling against Axis submarines and surface raiders. While the US is undoubtedly a
de facto ally of Britain, she has not crossed the line to become a
legal or
de jure ally of Britain in the war.
Germany is playing along with this little game too. As the events of the Second Happy Time showed, she can easily dispatch U-boats to US coastal waters and sink the cargoes being assembled there prior to be being convoyed across the Atlantic. Germany hasn't done that, or taken any other actions, which would provide FDR with a
casus belli because Germany doesn't want the US to fully participate in the war. The situation is fine to Germany as long as the US only nibbles around the edges.
So, the US does as much as it can without actually joining the war and Germany pretends not to notice as long as the US doesn't do too much and, apart from a few incidents like the
Reuben James, everyone is happy to dance up to the line and dance back again.
Then Japan shits in the punchbowl.
Now, as I seemingly must continually repeat, the US and Britain are
de jure allies in a war against Japan. Now the US has every legal right under international law to ship US supplies aboard US ships guarded in US convoys to a US ally in the war against Japan. The entire situation in the Atlantic has been completely changed. US forces aren't going stay in the Western Hemisphere or be content with turning over convoys to the RN/RCN at MOPP. US forces are going to hunt, attack, and sink any submarine they come across that threatens any US LOC with her ally in the war against Japan. And, because U-boats won't have huge fucking signs which proclaim
I'M GERMAN, REALLY! SO DON'T SHOOT BECAUSE YOU'RE ONLY FIGHTING JAPAN, OKAY?", German U-boats are going to be sunk by US warships anywhere US warships can find them.
Because German U-boats cannot tell War-Against-Japan supplies apart from War-Against-Germany supplies, they'll sink both. Because US warships cannot tell German submarines apart from Japanese submarines, they'll sink both too. And because both those things are going to happen, a war between Germany and the US cannot be prevented.
Japan's offensive makes a war between the US and Germany automatic. The only question left to answer is when that war will officially begin and Hitler decided to begin that war at a time of his choosing. He could officially start the war immediately while the US was still essentially unprepared or he could wait until later events in the Atlantic officially start the war when the US will be better prepared.
Japan left Germany with no choice about going to war with the US. The only choice left to Germany was when that war would begin.
So long as only Japan is formally at war with the Americans, there will necessarily be an effort for the U.S. to fight Japan to a much fuller extent sooner than was true in our timeline.
Do you even bother to read the many Pacific War threads here? The US cannot even consider fighting Japan to a "fuller extent" until the results the the 1940 Two Ocean Navy Bill begin sliding into the water in 1943.