according to this article and this one.

U.N. officials declined U.S. offers to provide tighter security at their Baghdad headquarters so they would have a friendlier image with the Iraqi public, American military officials said.

Coalition military forces did not provide security for the U.N. compound but patrolled the area, and one such patrol was nearby when a truck bomb exploded Tuesday, Pentagon officials said.

U.N. officials in Iraq deliberately decided to forgo strict security measures because the United Nations "did not want a large American presence outside," U.N. spokesman Salim Lone said.

Just weeks ago, U.S. forces in Iraq began erecting labyrinthine barriers around nonmilitary, "soft" targets in Baghdad to guard against bombings like Tuesday's at U.N. headquarters.

But of course, that must be Bush's fault.