America was never isolationist in terms of economics. The United States would trade with anyone in the world. However, there was a trend of being uninvolved in foreign affairs whenever possible. The Second War World transformed that, and the United States emerged as the leader of the Western "Free World", with military, political and economic interest and involvement in those nations and across the world. Since the end of the Cold War, it emerged as the hegemonic power of the entire world.
Generally, the concept of America as an isolationist nation is used in reference to an alternate WW2. However, I want to focus on the general concept of an America that is isolationist because it would be such an extreme contrast to the world as it came to stand in the latter 20th century and into the present day. Therefore, this is a discussion regardless of a "Nazi Victory" scenario should that be an issue, and I would frankly rather focus on a Second World War that the Axis loses regardless. I believe that would be a red herring distraction. Therefore, what if the United States had remained an isolationist nation up until the present day.