There are two scenarios that can produce this situation.
One scenario is that the USA and Germany do not go to war after Pearl Harbor because the two countries are allies. You get FDR out of the picture and a fascist sympathetic government in Washington. Or Hitler is able to come to terms with Britain. Or the Nazis declare victory after the fall of France, don't occupy northern France, don't bomb the UK, attack British shipping, or help the Italians so eventually Churchill is replaced, the British come to terms, and there is no need for lend lease, and then after Pearl Harbor Germany declares was on Japan, though I don't see why the Japanese do the Pearl Harbor attack in this situation, it probably completely changes the Japanese strategy.
The second scenario is that Germany just does not declare war on the USA after Pearl Harbor.
The second scenario has been debated in this board, and there is strong evidence that the USA was shortly going to declare war on Germany anyway. German inaction may not be enough, though their declaring war on Japan would at least confusing things.
Some version of the first scenario involves multiple unpredictable changes in the world situation.
With a straight up war between the US and Japan, the US army is not that useful in an island-naval war, and with only air supply to Nationalist China possible there ithey can't send much there, even assuming Chiang would make good use of any additional material. And the Americans will always keep a fleet in the Atlantic regardless and in the actual war they didn
t have a problem with transferring assets to the Pacific as needed. Really nothing changes.
Despite Roosevelt's publically stated "Germany first" policy, if you look at exactly where Washington directed its forces, particularly transports, it really was "Japan first" through 1943.