WASHINGTON — In late 2002, two strong-willed CIA officers, identified only as Beth and Margaret, were at daggers drawn. They had diametrically opposing views about the veracity of an Iraqi defector’s reports concerning Saddam Hussein’s biological weapons programs, and especially the notorious but never seen mobile weapons labs.

“Look,” said Beth defiantly, “we can validate a lot of what this guy says.”

Margaret, angry and incredulous: “Where did you validate it?”

Beth: “On the Internet.”

Margaret: “Exactly, it’s on the Internet. That’s where he got it too!”

Margaret was right in that episode, recounted in the new book “Curveball” by Bob Drogin of the Los Angeles Times. Curveball was the code name of the Iraqi defector in Germany on whose reports the Bush administration relied heavily in its argument that Saddam’s weapons of mass destruction justified a preventive war.

In 1999, Curveball defected to Germany, which has a significant portion of the Iraqi diaspora. Seeking the good life — a prestigious job, a Mercedes — he jumped to the head of the line of asylum-seekers and got the attention of Germany’s intelligence agency with the word “Biowaffen,” germ weapons. He claimed to have been deeply involved in Saddam’s sophisticated and deadly science, particularly those notorious mobile labs. Notorious and, we now know, nonexistent.

German intelligence officials — partly because intelligence agencies are like this and partly because they thought Germany had been unfairly blamed by the United States for not detecting the Hamburg cell from which three of the four 9/11 pilots came — refused to allow U.S. officials to interview Curveball. Yet by March 2001, the Germans were expressing doubts about him; by April 2002, the British were, too.

So were some U.S. officials, such as Margaret. But others became invested in Curveball’s credibility, and soon they could not back down without risking personal mortification and institutional disgrace — both of which came, of course, after the invasion. Then some of Curveball’s Iraqi acquaintances were located and identified him as a “congenital liar” who was not a scientist but a taxi driver. But before the invasion, he supplied an important rationale for launching it: He was the most important source for Colin Powell’s 80-minute address to the U.N. Security Council detailing Iraq’s WMD programs, the address that solidified American support for war.

“We have,” said Powell, “firsthand descriptions” of “biological weapons factories on wheels and on rails.” Powell took the word of people who took Curveball’s word. Such as Beth, who had conceded that Curveball was odd, but weren’t most defectors? Curveball’s reports were “too detailed to be a fabrication,” and too complicated and technical for Margaret to judge. “Well,” Margaret replied, “you can kiss my ass in Macy’s window.” And the war came.

Drogin’s account of the search for WMDs after Baghdad fell would be hilarious were the facts not scandalous and the implications not tragic. That missile spotted by analysts of satellite imagery? It was a rotating steel drum for drying corn. The missile photographed from the air? Chickens in Iraq are raised in long, low half-cylinder coops. Some weapons searchers finally had T-shirts printed with the U.N. symbol and the words “Ballistic Chicken Farm Inspection Team.” In the middle of the night in Baghdad, Vice President Cheney’s chief of staff, Scooter Libby, was calling from Washington with precise geographic coordinates to guide searchers to Iraq’s hidden WMDs. The supposed hiding place was in Lebanon.

Drogin’s book refutes its subtitle, which is: “Spies, Lies and the Con Man Who Caused a War.” Curveball did not cause the war; rather, he greased the slide to war by nourishing the certitudes of people whose confidence made them blind to his implausibility.

Drogin probably overstates his indictment of U.S. officials when he says that the CIA, having failed to “connect the dots” prior to 9/11, “made up the dots” regarding Iraq’s WMDs. In the next paragraph his assessment is less sinister — but more alarming. More alarming because his formulation suggests that the problem was human nature, and there is always a lot of that in government. Calling Curveball a fabricator, Drogin writes, “implied that U.S. intelligence had fallen for a clever hoax. The truth was more disturbing. The defector didn’t con the spies so much as they conned themselves.”

Drogin’s book arrives, serendipitously, as some Washington voices, many of them familiar, are reprising a familiar theme — Iran’s nuclear program is near a fruition that justifies preventive military action. Whether or not these voices should be heeded, Drogin’s book explains one reason why they will not be.

George Will is a Washington Post columnist. His e-mail address is georgewill@washpost.com.