Someone correct me if I am wrong, but because Germany, Italy and Japan were all allies. When the US declared war on Japan, Germany and Italy due to the terms of their agreement declared war on the US.

The US always planned a Europe First policy in the event of a war. Japan could have a temporary run of Asia until the Allies were able to gain control of Europe, from which point of strength they could address the Japanese situation.

However the kicker is that even with Pearl Harbor, going to war in Europe was not a foregone conclusion. Similar to the criticism against the war in Iraq, people would question why we declared war on a nation that didn't attack us.

Fortunately that was never an issue since the Germans declared war for us. The question is whether or not the war plan Hitler cited in his speech to the Reichstag was deliberately leaked to influence his decision. Three days before Pearl Harbor, a paper in Chicago published details of the Rainbow 5 plan. The plan was an updated version of the old colored war plans of the interwar period to account for the rise of fascist power. By 1943 the US envisioned invading North Africa, following it up with an invasion of Italy and ultimately fighting Germany. That's quite similar to what actually happened in the war.

This had a large part do with with US/British diplomatic relations. After Pearl Harbor it wasn't a forgone conclusion that the US would be at war with Germany. But Churchill has close ties to the US and worked hard to bring the US into the European theater. After Pearl Harbor, the British actually declared war on Japan before the US did because of Churchill's efforts behind the scenes.