The three common mistaken impressions are that:
-U.S. forces found weapons of mass destruction in Iraq.
-There's clear evidence that Iraqi President Saddam Hussein worked closely with the Sept. 11 terrorists.
-People in foreign countries generally either backed the U.S.-led war or were evenly split between supporting and opposing it.
Overall, 60 percent of Americans held at least one of those views in polls reported between January and September by the Program on International Policy Attitudes, based at the University of Maryland in College Park, and the polling firm, Knowledge Networks based in Menlo Park, Calif.
"While we cannot assert that these misperceptions created the support for going to war with Iraq, it does appear likely that support for the war would be substantially lower if fewer members of the public had these misperceptions," said Steven Kull, who directs Maryland's program.
In fact, no weapons of mass destruction have been found in Iraq. U.S. intelligence has found no clear evidence that Saddam was working closely with al-Qaida or was involved in the Sept. 11, 2001, attacks. Gallup polls found large majorities opposed to the war in most countries.
PIPA's seven polls, which included 9,611 respondents, had a margin of error from 2 to 3.5 percent.
The analysis released Thursday also correlated the misperceptions with the primary news source of the mistaken respondents. For example, 80 percent of those who said they relied on Fox News and 71 percent of those who said they relied on CBS believed at least one of the three misperceptions.
The comparable figures were 47 percent for those who said they relied most on newspapers and magazines and 23 percent for those who said they relied on PBS or National Public Radio.
The reasons for the misperceptions are numerous, Kull and other analysts said.
They noted that the Bush administration had misstated or exaggerated some of the intelligence findings, with Bush himself saying in May: "We found the weapons of mass destruction ... and we'll find more as time goes by."
The Bush administration has also been a factor in persistent confusion.
Last month, for example, Bush said there was no evidence that Saddam was involved in the Sept. 11 attack after Vice President Dick Cheney suggested a link. Cheney, in a "Meet the Press" interview, had described Iraq as "the geographic base of the terrorists who had us under assault now for many years, but most especially on 9-11."
Why some news audiences had more accurate impressions than others was less clear.
Kull cited instances in which TV and newspapers gave prominent coverage to reports that banned weapons might have been found in Iraq, but only modest coverage when those reports turned out to be wrong.
Susan Moeller, a University of Maryland professor, said that much reporting had consisted of "stenographic coverage of government statements," with less attention to whether the government's statements were accurate.
The study found that belief in inaccurate information often persisted, and that misconceptions were much more likely among backers of the war. Last month, as in June, for example, nearly a quarter of those polled thought banned weapons had been found in Iraq. Nearly half thought in September that there was clear evidence that Saddam had worked closely with al-Qaida.
Among those with one of the three misconceptions, 53 percent supported the war. Among those with two, 78 percent supported it. Among those with three, 86 percent backed it. By contrast, less than a quarter of those polled who had none of the misconceptions backed the war.