Nothing that happens in human society is inevitable. There are always things that countries could have done differently to avoid war. For example, it is not as if the US absolutely had to go to war after the Pearl Harbor attack. It could have pulled its military back to the West Coast and simply ridden out the war. It is not at all likely any American administration would ever take that choice, but it was a choice. Nothing of this sort is inevitable.
The US involvement in the war was not inevitable. The US had stayed out of the war fairly well up until the Pearl Harbor attack. If the US had not been so strongly opposed to Japan having an empire in the Western and Central Pacific, the attack would not have happened. If the Japanese had not been so set on getting an empire, the US would not have had to oppose them. Either of these circumstances would have prevented US involvement in the war.
US involvement in the war was likely, but it was by no means inevitable.
Obviously, President Roosevelt and Congress had a choice in whether the US would enter the war or not. The issue, however, is not as simple as that. One should also consider the repercussions if the US had remained neutral. It was pertinently obvious that the Axis powers, constituting Germany, Italy and Japan, had been growing more powerful and were, in fact, winning the war. Their aggressive campaigns lead to the destruction of huge areas, the deaths of millions, and the creation of a world of uncertainty and chaos. Furthermore, the three nations' fascist policies meant that victory for them would introduce extremely racist and discriminatory policies, though it should be noted that the was already afflicted by widespread racism.
The three leaders had all adopted dictatorial stances and their tyranny would have subjected many to oppression and enslavement. Such policies were out of tune with developments toward greater freedoms and democracy throughout the world. A victory for the Axis powers would reverse everything that so many had fought and died for.
The world needed some intervention to turn the tide against the 'Axis of Evil.'
The United States had adopted a neutral stance and refused to enter the conflict. It was, however, indirectly involved by providing the Allied Forces with arms and ammunition on a 'pay as you go' basis. Sentiment against Hitler, Mussolini and their policies had grown in the US and the public had reached a point where they believed that something had to be done. The US still maintained its policy of 'non involvement,' though. President Roosevelt, in the initial stages of the war, would not be able to persuade Congress to declare war since it strongly believed in maintaining US neutrality. The President believed, though, that some form of intervention was needed to stem the tide of devastation and disaster. The US then provided aid to destitute countries, such as China, through the Lend-Lease Act.
Japan had grown confident of its ability to expand its influence in the Pacific region. It had the backing of its two powerful allies and wished to access the rich natural resources in the region since the US and other nations had cut its supplies. Japan believed that domination of the region would lead to greater power. In order to achieve this, Japan decided to weaken US navy capabilities in the region, leading to the attack on Pearl Harbor on December 7, 1941.
President Roosevelt then had enough reason to declare war and, with the almost unanimous support (there was one vote against) of Congress, declared war on Japan on December 8, 1941. In support of their ally, Germany and Italy declared war on the United States on December 11, 1941. US entry into the conflict turned the tide against the Axis Powers and eventually resulted in victory for the Allied forces.
All things considered, it seems that the US was driven to participate by an inescapable choice: allow the Axis Powers to dominate, oppress and tyrannize the world or do something to stop the advance of evil. Surely, without its intervention, the planet would have entered an era of unimaginable malice.