In his March 17th ultimatum Bush left no doubt that Saddam Hussein's failure to disarm has mandated his removal from power on his own accord within forty-eight hours or by the United States military "commenced at a time of our choosing." This action was justified as the Iraqi regime "continues to possess and conceal some of the most lethal weapons ever devised."

Saddam Hussein never fliched and the United States unleashed its military might. American troops rolled into Iraq equipped with chemical and biological weapons protection. Troops were instructed to search for weapons of mass destruction. Soon after the campaign began, General Franks indicated coalition forces would engage in what he deemed "sensitive site exploration." This activity would continue "as we go along."

On March 23rd, acting on an intelligence tip, the Third Division searched a conventional weapons depot that was claimed to have housed chemical weapons as well. The search came up empty.
Source: CBS News

On March 25, a captured suspected chemical plant was searched for evidence of illegal weapons. The plant was inspected by American authorities and initial indications were that it had not been used to create weapons since at least 1998, five years ago.
Source: CBS News

On March 29, US forces searched for weapons of mass destruction at an Ansar al-Islam camp in northern Iraq. The camp was said to have possible ties to al qaeda. Intelligence officials were convinced they'd locate ricin, a chemical more deadly than cyanide. The operation was planned prior to the war. The camp was bombed. Special forces moved in and scoured it. They found no weapons of mass destruction, no ricin.
Source: ABC News

On April 4, the Third Infantry captured a walled Iraqi compound in Musayyib, a city unique for containing two Ys in the middle of its name. Troops investigated an arms cache and found gas masks and vials of white powder. The United States had the vials tested for evidence of chemical weapons. The powder was a conventional explosive.
Source: Washington Post

On April 7, the US Ministry of Propaganda, also known as the New York Post, blared a headline to the effect that the United States had discovered chemical weapons in Iraq. (I can't quote it since they appeared to have pulled it from their website.) Initial test indicate that the 14 barrels may contain nerve agents or mustard gas. The Post's bosses in the Bush Administration were more cautious and urged patience until testing was completed. Field sensors can often trip false negatives as nerve agents chemically similar to widely-used organophosphate insecticides such as malathion.
Source: New Scientist

Coalition forces have been actively searching for banned weapons since the war began. The fourteen barrels unearthed in an insecticide plant are perhaps the most promising evidence of illegal weapons. But on April 9, the Hussein government collapsed, Baghdad was captured and the Iraqis never attempted to use any weapons of mass destruction. If Iraq failed to use them to avert an invasion, the United States finally in its limited range, then can a case be made that they posed a "serious and mounting threat to our country, and our friends and our allies?"

The diplomatic and legal justifications for this war lie in the contention that Hussein hid weapons of mass destruction. If the Bush Administration fails to produce them, then its credibility is destroyed and the blood of this war lies smeared on its hands. Its international standing was already strained for failure to produce justification after a series of false positives. Complete failure to do so would be completely unacceptable.