War between the US and the Axis was inevitable with Roosevelt as president, even during a "divided" Congress, since his administration stopped at almost nothing to set the conditions for a confrontation to be inevitable. In retrospect, it's a shame that it took Pearl Harbor for Americans to do the right thing and join the Allies sooner, like following the invasion of the USSR, but it's very likely America would have joined the Allies before the end of 1942.

Let's look at the facts. The USS Niblack is famous as the first American warship to conduct hostile actions during WWII, but this took place well before the US entered the war officially.

On April 10, 1941, it was patrolling off the coast of Iceland and detected what the captain believed was a u-boat, where upon he fired depth charges until the u-boat seemed to depart the area. Then in July, the Niblack departed Newfoundland, Canada to escort US ground forces that would replace British occupation forces in Iceland (nominally still part of Denmark). Finally, in October, it escorted the convoy in which the USS Rueben James was sunk by U-522.

Now think of it this way. A neutral country is not only supplying your enemies with munitions and not you, but they are augmenting your enemies’ naval and ground forces so your enemies have an easier time fighting you. That's not going to make you happy with that neutral, and sooner or later, you're going to take a punch at them. That's exactly what the Roosevelt Administration was thinking too, especially when it publicly agreed to the Atlantic Charter with Great Britain in August 1941.

The 8 points of the agreement read similarly to Wilson’s idealistic 14 Points from WWI, and make it clear that the neutral US sided with the Allies against the Axis. In fact, British bombers dropped copies over German cities to make sure they understood this. In neutral Japan, existence of the agreement supported the militarists’ policy for a more aggressive approach towards the neutral US and the British since they were obviously aimed against Japan’s war in China too. The 8 points later served as the basis of the United Nations charter.

Given these circumstances, Pearl Harbor didn't really matter, since time was running out on a neutral US. Recall also that US bases in Guam and the PI were attacked by Japan at the same time as Pearl Harbor, and would probably still have occurred even if bad weather somehow prevented a strike against Pearl Harbor. Time was running out for Japan’s dwindling raw materials due to Roosevelt’s full embargo, so they likely would have attacked something important to the US to alleviate the stranglehold. It was only a matter of time and the right set of circumstances, which Germany and Japan would be eager to provide, before Roosevelt eventually had a solid case for war.

Otherwise, if the US was really as divided as popular belief says, then Roosevelt should've been impeached before Pearl Harbor, but that's not the case at all. Just reading the headlines from newspapers will tell anyone the US public understood that a war was coming, but just didn't like the reasons being given. Pearl Harbor definitely removed that problem, but the administration was doing as much as it could in both the Atlantic and Pacific to make it difficult for Germany or Japan to ignore its presence in their “spheres of influence.” War would've come before 1942 ended since Japan was running out of resources before then, and US support for Britain would’ve continued to be extended through 1942 until American ships and planes sunk u-boats on a regular basis in defense of the Atlantic convoys, forcing Germany to unleash the Wolfpacks at last.

However, without a Japanese attack on the US in December 1941, then the Japanese would have had to either chosen to forego the non-aggression pact with the USSR and joined the German invasion, or agreed to US demands to remove troops from French Indochina and wind down the war in China.

Both scenarios are possible, but less likely than attacking the US. In the first case, the Japanese military’s respect for Soviet fighting capacity along with the tacit neutrality of Mao’s communist guerrillas in China suited a seizure of Allied and US bases; second, the Roosevelt Administration did not attempt to sincerely negotiate with Japan, even when Tokyo was seriously considering a negotiated settlement with Chaing Kai-shek. The latter impasse resulted in the fall of the Japanese Government in October 1941 and appointment of General Tojo for the express purpose of attacking the Allies and the US.

So, no matter how much armchair historians want to believe a magical barrier of “splendid isolation” enveloped the US and “protected” it from all outside interference until December 7, 1941, it's quite apparent that we have always lived in a dynamic system of interconnected decisions, where every action inevitably has a reaction, even if unexpected, and this fact was not lost on the leaders of the Axis and Allied governments, as well as the Roosevelt Administration, especially when pushing the limits of US “neutrality.”