Im sorry, I just dont think that Isolationism is effective for a nation. Especially not one as dependent as the US is on foreign "stuff" (for lack of a better category). And I do think that this can change as well. But even if the US becomes self-sustained in terms of energy/commerce/etc. we still have ties to the world.
In the case of Iraq specifically, I think that the UN was horribly inefficient. However, I think that the US could have allowed inspection to continue. And the US could have applied more pressure to its allies in the UN to get the process sped up or tightened up. I just wonder, if we had given them 3 more months and they had come forward to say they had no weapons, and no legitimate components/plans/etc. would we have been satisfied. Would Bush have blazed his path into the Middle East anyways? All hypotheitical, and at this point, truly not worth entertaining beyond that.
I think that the best thing for the US would be to invest more time and effort into the UN. Apply pressure to our allies to get the UN shaped into a tool we can agree with. As a concept, the world needs the UN. In practice, as with many things, the reality falls short of the ideal.