Why the leaders of the United States have this obsession of
"nation building"? South Vietnam and now Iraq and the noises
they make about Japan and Germany?

Well, they didn´t do any "nation building" in Japan and Germany.
Those nations existed before the Allied occupation; that was the
reason of the success. In Japan, the government and byrocracy
were pretty much intact and McArthur could take place in an
existing system of power. Germany was more shambles, but it
too was a nation and it was a totally and utterly defeated nation:
It people wanted only peace and they were handled pretty
graciously by the occupiers considering what Germany had done.

South Vietnam had no own identity. It was a fiefdom carved for
themselves by people mainly from the area of North Viernam,
usually Catholics when the South Vietnamese were usually
Buddhists. No history, nothing that would have separated it from
the North, except an elite minority, which the United States
supported.

And why did the US support South Vietnam?

Well of course there was the rabid fear of communism, ignorance
on the situation in Vietnam and the neighbouring countries. US
leadership thought they knew what kind of impact a unified,
communist Vietnam would have, when in fact they knew very
little. Bluntly speaking, they had very wild imaginations. But still,
I think that there was still a one reason: Ambition and fame.

"Nation building" seems to have an odd attraction for American
politicians, when most think that it is a heavy burden. I personally
think that the challenge itself has been one reason that has driven
American policy makers. After all, to be able to show a "new nation"
as a memorial of your presidency would be quite spectacular.
And when you and your advisers have as exact knowledge about
the situation and problems concerning "nation building" as
Columbus had about the size of the Earth, then the challenge
may become too tempting.

It´s not uncommon that American foreign policy has often become
a testing ground of presidents own pet projects; but in Bush´s
administration one feels that it´s not the president that is behind
"free Iraq" and "democratic Middle East". Plainly speaking, he is too
dumb to be accused of it.