A credible witness has stepped forward, and the time is ripe for a full inquiry into what must be regarded as a great uranium hoax. The witness, mentioned but not named until now, is a veteran career foreign service officer and ambassador named Joseph C. Wilson 4th. He told his story in The New York Times Sunday.

The incident involves a startling new piece of evidence that President Bush cited in his State of the Union address on Jan. 28 to show that Iraq was developing a nuclear arms program. Mr. Bush said: “The British government has learned that Saddam Hussein recently sought significant quantities of uranium from Africa … Saddam Hussein has not credibly explained these activities. He clearly has much to hide.”

The British had tipped Vice President Dick Cheney a year earlier that Iraq had bought uranium yellowcake – a lightly processed ore – from Niger in the late 1990s. Records including a memorandum of agreement purported to document the sale.

Mr. Bush made timely use of this supposed new evidence. The Bush administration had decided in late 2001, right after the campaign in Afghanistan, to invade Iraq. By January 2002, U.S. troops were massed in Kuwait, but the French and Germans were leading world opposition to the war plan, U.S. intelligence services were split over whether Saddam posed a nuclear threat, and polls showed that American support for the war, though still strong, was slipping somewhat.

Ambassador Wilson writes that officials at the Central Intelligence Agency told him in February 2002 that Mr. Cheney’s office had questions about the intelligence report. They asked him to travel to Niger to check out the story so they could respond to the Cheney query.

Mr. Wilson arrived in late February 2002 in the Niger capital, Niamey, and met with the U.S. Ambassador, Barbro Owens-Kirkpatrick. She told him she knew about the allegations of uranium sales to Iraq and felt she had already debunked them in her reports to Washington. He spent eight days checking with dozens of current and former government officials concerned with the country’s uranium business. He writes: “It did not take long to conclude that it was highly doubtful that any such transaction had ever taken place.”

The two uranium mines were run by an international consortium and monitored by the International Atomic Energy Agency. Mr. Wilson said that “there’s simply too much oversight over too small an industry for a sale to have transpired.” Other investigators had shown that the documents had glaring errors, including signatures by officials who already had left the government, and were probably forgeries.

Mr. Wilson briefed the U.S. ambassador, and, on his return to Washington in early March 2002, reported his findings to officials at the C.I.A. and the State Department. He assumed that the C.I.A. told the office of the vice president and considered the matter settled.

But it wasn’t settled at all. In September 2002, a British government “white paper” asserted that Saddam Hussein and his unconventional arms posed an immediate danger. As evidence, the report cited Iraq’s attempts to purchase uranium from an African country. Then, in January, came President Bush’s speech repeating the British charges.

Ambassador Wilson now wants to know how his answer to the vice president’s question was or was not used by “our political leadership.” He concedes that his information may have been deemed inaccurate. “If, however, the information was ignored because it did not fit certain preconceptions about Iraq, then a legitimate argument can be made that we went to war under false pretenses.”

Another question is whether Mr. Wilson’s report ever reached Vice President Cheney. And if it didn’t, why not? And how in the world did a phony piece of intelligence get into the State of the Union address?

A decision to go to war is the most serious question faced by any government. It must be based intelligence that is sound and properly circulated. When went wrong here is an urgent and obvious question for the Senate Intelligence Committee. And Ambassador Wilson should be a key witness.