LONDON - In March, 35 American soldiers and Marines died in Iraq, nearly all of them killed by insurgents.
In April, 52 Americans were killed.
The total in May was 80.
In those three months, insurgents killed more U.S. troops than Saddam Hussein's army did during the fight to oust his regime.
As of Friday, 1,726 uniformed Americans had been killed in Iraq since the war began in March 2003, and all but 139 of the deaths - 1,587 of them - have occurred during the insurgency.
The insurgency's lethality has been demonstrated even more dramatically by its targeting of Iraqi civilians and police officers. According to the Iraq Body Count, a volunteer organization of researchers and academics, at least 16,816 Iraqis were killed in the insurgency by the end of May, going back to May 1, 2003, when President Bush declared an end to major combat operations.
The past four months have been among the deadliest for Iraqis. Last month, 653 Iraqis were killed, an average of more than 21 a day.
"Nothing could show more clearly than the number of people being killed just how the war is going," said Peter Zimmerman, an American-born professor in the War Studies department at King's College London. "Those numbers say what nobody wants to - that we in fact do not have the insurgents on their last legs."
Defense Secretary Donald H. Rumsfeld told Congress on Thursday that the United States would prevail despite the increasing number of deaths.
"Any who say that we've lost this war, or that we're losing this war, are wrong - we are not," Rumsfeld said. "We have recognized this is a tough business. It is difficult, it is dangerous and it is not predictable."
Measured against past wars, the number of U.S. service members killed in Iraq is relatively low. The war in Korea claimed more than 54,000 U.S. troops; 58,000 died in Vietnam.
In Iraq, an average of five U.S. military personnel have been killed every two days since the start of the year.
The evident dangers and the lack of a clear end to the conflict have damaged the U.S. military's ability to recruit new volunteers, according to Pentagon figures, and, as several polls indicate, reinforced negative attitudes in Europe and the Middle East about the United States.
The most recent monthly totals for the American and Iraqi deaths are part of an overall upward trend in casualties, indicating that the U.S. military has encountered stronger resistance than first acknowledged by the Bush administration, which has been predicting the imminent defeat of the insurgency for more than two years.
Vice President Dick Cheney, said twice recently that the insurgency was "in its final throes."
The military's top commander in Iraq, Gen. John P. Abizaid, speaking to Congress on Thursday, reported that the insurgency was as strong as it was six months ago.
Based on trends apparent in monthly tallies of the dead, the insurgency has killed more effectively during its second year than in its first, and since April has become increasingly deadly, after a two-year pattern of several active months followed by a brief period of relative quiet.
The Bush administration has resisted calls for setting a timetable to begin bringing troops home and has emphasized efforts to train an Iraqi army.
As more Iraqi battalions become stronger, the strategy calls for gradually withdrawing American forces.
"The report from the field is that while it's tough, more and more Iraqis are becoming battle-hardened and trained to defend themselves," Bush said last week. "And that's exactly the strategy that's going to work."
The insurgents, however, have killed increasing numbers of civilians, even as larger numbers of Iraqi troops were supplementing the 135,000-strong U.S. force in the country.
Because of the nature of the insurgency, nobody seems certain how many fighters there are.
"There's no way of knowing who all of them are, but it seems as though the biggest group is disaffected Sunnis," said Joost Hilterman, director of the Middle East Project of the International Crisis Group, speaking from Jordan.
Others include foreign fighters and former regime members, he said.
"They won't give up," he said. "The foreign fighters, especially, have been drawn to Iraq because it's an opportunity to go after Americans face-to-face, and they're willing to die to take their shots."
The insurgents have succeeded partly by identifying and exploiting U.S. military weaknesses - and then adapting their tactics to U.S. defenses.
Last month, for example, there were reported to be about 700 attacks against American forces using improvised explosive devices, or IEDs, the highest number since the invasion of Iraq.
The insurgents have relied on the explosives since early in the fight, taking advantage of the Defense Department's failure to provide troops with an adequate number of armored vehicles.
When more armored vehicles arrived, the insurgents were able to quickly and significantly alter bomb designs to create blasts capable of penetrating the armor.
And when the military began using jamming equipment to prevent bombs from being detonated by radio waves, the insurgents moved to infrared lasers.
"This is a serious counter-insurgency campaign that the administration was late to take seriously, and we're paying the price," said Anthony Cordesman, a fellow at the Center for Strategic and International Studies and a former national security adviser to Sen. John McCain. "The best we can say is that we're not losing. But the truth is we can't say we're winning."
The insurgency was able to form quickly, he said, because U.S. war planning seemed to have extended only as far as toppling Hussein and having the U.S. military occupy Iraq.
Despite a steady series of attacks against U.S. troops in the early days after Hussein's fall, the Bush administration failed to react with a major show of power, according to Cordesman.
"The single biggest problem was they made no real preparations for stability operations, which allowed for chaos in Iraq, and that allowed the insurgency to build in numbers and effectiveness," he said. "That doesn't mean the administration can be blamed for all that's gone wrong, because nothing like taking over Iraq had happened for us before.
"You would have expected, though, at minimum, a quicker reaction once it was obvious the war didn't end with Saddam's fall."
As the insurgency gained steam, the Bush administration began stating its hopes that any of a series of events would bring the violence to an end.
On at least five occasions, the administration described events in Iraq as marking the insurgency's end: the creation of the Iraqi Governing Council; the killing of Hussein's sons, Uday and Qusai; the Coalition Provisional Authority's handing over its powers to an Iraqi Cabinet; the capture of Hussein himself; and January's parliamentary elections.
In each case, the events were followed by a regrouping and strengthening of the insurgency.
"I think 'seeing the light at the end of the tunnel' is something we learned to discount during the Vietnam War," said Zimmerman, the King's College professor. "Yet, that line has been trotted out time after time."
On Friday, while saying that the insurgency would not end immediately, Bush said at a news conference that creation of Iraq's new constitution, which is to be written by Aug. 15, will help bring stability to Iraq.
"Success will happen in Iraq when the political process moves forward - like it is," Bush said.
A CBS News/New York Times poll this month found that 59 percent of Americans felt the war was going poorly, and just 37 percent approved of Bush's handling of Iraq. A Gallup poll showed six in 10 Americans favoring full or partial withdrawal of U.S. forces.
Sen. Chuck Hagel, a Republican from Nebraska who recently returned from a tour of Iraq, told U.S. News & World Report this month: "Things aren't getting better; they're getting worse. The White House is completely disconnected from reality. The reality is we're losing in Iraq."
Cordesman, the former McCain adviser, said he returned from Iraq two weeks ago.
"I talked to a lot of U.S. military, and there was no perception from anybody - from anybody - that the insurgency was being defeated," he said. "They weren't at all optimistic that this thing is coming to an end any time soon."