During WWII, there was less doubt about which side the US would join. War with Japan had been anticipated for years and a strategy of waging war adopted in the 20s called War Plan Orange. With Japan's expansionist policies, and the US' territories in the pacific (Guam, Philippines, Wake) it was more a matter of when than if.
The US considered Germany less of a direct threat. Germany was not a great maritime power, seaborne invasion was not a threat. With no territorial interests in Europe, US had no direct stake in the war. There was a very strong isolationist movement in the US which Roosevelt had to carefully step around to supply the Allies. The more the US got involved in supplying arms to the Allies, the more they came into conflict with German U-boats, the more likely war became.
However, there is a great question of when the US would have entered WWII. It likely would not have been December 1941. The US military was grossly unprepared for war. In 1940 it was largely a reserve force of a few hundred thousand. Due to the delay in rearming and the chaos caused by rapid expansion, Army divisions were still being assembled, trained and brought up to strength. They were far from ready for the complicated logistical task of deploying overseas. The first viable, but rushed and flawed, US medium tank the M3 Lee, was not in production until August 1941. The ubiquitous M4 Sherman would not roll off production lines until 1942. The US Navy, while prepared to fight a conventional war in the pacific, was unprepared for defensive submarine warfare as shown in Operation Drumbeat.
The Attack On Pearl Harbor was a strategically decisive defeat for the Axis in many ways. It forced the US to enter the war early, removing the need to maintain a veneer of neutrality for its own citizens and ramping up production earlier than it would have otherwise. It over strained the Japanese military who were now attacking in every direction, had they waited they could have fought a more concentrated defensive war with preparations, or no war at all!
Second, and this is often overlooked, Pearl Harbor caused Germany to declare war on the US. It is often assumed it was the other way around, that the US automatically declared war on Germany, not the case. There was a great risk of having the US fight Japan and, wanting to put all its attention and resources into that war, not fight in Europe. Germany's declaration of war eliminated the possibility for the US to be knocked out of the war in Europe without firing a shot.