The United States always viewed itself as an ideologically superior force, built on its premise of "Life, Liberty and Pursuit of Happiness" as shown in the U.S. declaration of independence.

Now, to an outsider like myself, the American approach to creating this narrative is somewhat hypocritical considering elements of U.S. history like slavery, segregation, the treatment of Native American tribes in historical proceedings like the Trail of Tears and so on, but all countries have the desire to paint themselves as somehow morally superior and, more importantly, morally justified in their political actions.

When it comes to the United States in particular, this need for self-justification might indeed be considered hypocritical, but it has always been one thing: Consistent. Here you can see colonial peoples of Hawaii, Cuba and the Philippines celebrating their liberation by American forces after the War against Spain in 1898. Here is the Lady Cuba begging the masculine and heroic Uncle Sam to save her from the villainous black-clad Spainiard in a caricature of 1896.

Let's go to 1917 as the next time in this era of the United States that the U.S. went to war. Again, we heavily see the American artists putting the U.S. as the agent of liberty. This is important: The United States never acts on the world stage for purely self-serving reasons, only ever to preserve what is right as per the American codex. Chief cause of American action is of course the one, the only: Liberty. Here immigrants are asked to buy U.S. "liberty loans". Here the liberty loan stomps on the authoritarian German enemy. Here you have "Liberty Bonds" as a measurement against the freedom-hating and barbarian German "Hun".

Okay, I'll get to the point here now, especially considering that U.S. history was never my strong point, much more focussing on Germany and France between about the 1860s and the 1980s (which of course is a period of time in which both countries were heavily influenced by and did heavily influence the USA). But what you have to understand is that the U.S. was by no means isolated from the rest of the world or from its own history as they entered World War 2. The recurring theme of ideology, as you describe it, was very much present in American culture and American self-portrayal well before Mussolini made fascism a force to be reckoned with in 1922 and still well before Lenin brought communism to the attention of the world in 1917.

The United States, as you know, chose to remain neutral in the early years of the war, even though especially the ruling Democratic Party around Franklin D. Roosevelt became increasingly pro-Allies between September of 1939 and December of 1941.

Roosevelt, upon winning his historic third term (the term for 1941-45) in office in the election of 1940, chose to further American readiness for war and American military materiel support for the cornered British Empire. Which term did he choose to refer to America's war-important heavy industry in December of 1940? "The great arsenal of democracy". Yeah.

So, the United States, even while still clearly committed to its armed neutrality, was going "the Hun and the Jap better don't force us to join this war or we will bring them some sweet democratic freedoms straight to their doorstep".

A tougher sell then of course was the second-biggest recipient of American aid: The Soviet Union. Why not let the totalitarian superpowers duke it out? Whoever won the "Nazis vs Soviets" struggle would be a pain in the butt for American ideals of liberty, right? Well, check out this well-known wartime propaganda poster. The United States managed to even sell the support for Stalin's mass-murdering regime as a necessary effort to support the peoples fighting for freedom. The definition of "freedom" would be the next interesting question, but I will not go there for now. You can listen to Dan Carlin's "The Celtic Holocaust", where in the beginning he goes on about the various definitions of freedom a person can have, although he does it in the context of Julius Caesar's conquest of Gaul.

So, to answer the question you originally stated:

"When was the storyline of WW2 as an ideological war created in the USA?"

In 1776, if I were to take a guess. The United States from the very beginning saw its own existance and its position on the world stage as an ongoing ideological struggle, fighting for life, liberty and the self-determination of all peoples*.

* Exceptions may apply. If you are the native people of a U.S. colonial territory, you might not qualify for Freedom™ or U.S. presidential elections. If you are a non-white people living before the 1950s, you might not qualify for Freedom™. Further restrictions may apply.

Okay, that last joke aside, I hope I helped answer at least part of the question.