Are you sure you have the right War? The United States did not adopt an isolationist policy after WWII. The occupation of Europe and Japan, the Korean War, The Vietnam War, and the global projection of military power are not the actions of isolationism.
I believe you will have better luck writing about US isolationism after WW I. Following that ';War to end all wars'; the US did withdraw. The US even refused to Join the League of Nations despite the fact that President Wilson pushed hard to start that precursor to the United Nations.Why did usa adopt the policy of isolation in the time after ww2?
Maybe the answer is because they didn't?