Vietnam. Indeed, unlike with Saddam post-1991, we might have actually been able to work with Ho Chi Minh. After all, as far as I know, Ho Chi Minh was either as much or almost as much of a nationalist as he was a Communist.
Apparently, Ho Chi Minh sent some sort of letter of good will or a request/offer for/of aid to DC during the Truman regime that was (needless to say) ignored.
Well, LBJ also sent two letters of good will in 1967, offering a total suspension of the air strikes if the North Vietnamese stop operating in the South. It was rebuked like many other diplomatic initiatives (like from the pope). They only agreed to negotiate in 1968 after they lost tens of thousands of men during the Tet offensive and after Johnson declined to seek reelection. It took them another two months to agree on a negotiating place, after the LBJ Administration made several proposals; mostly on neutral territory.
True, it was a mistake that both Woodrow Wilson and Harry Truman rejected Ho's requests (or didn't even talk with him), but don't forget that the North Vietnamese were very stubborn during the Vietnam War. But it may very well be, that Ho's influence declined during the mid/late 1960s and that others made these decisions which prolonged the war and cost thousands of people their lives.