But Rumsfeld said the administration of President George W. Bush and its coalition allies have no plans to withdraw from Iraq in the face of increased resistance from pockets of both Sunni and Shi'a insurgents.
"The United States will stay the course," Rumsfeld said. "We will stay until the task is complete. As President [George W.] Bush has said, we did not charge hundreds of miles into the heart of Iraq and pay a bitter cost of casualties to liberate 25 million people, only to retreat before a band of thugs and assassins."
The violence so far this week is believed to have killed more than 200 Iraqis, 35 U.S. soldiers, and two coalition soldiers -- a Ukrainian and a Salvadoran. Rumsfeld called the current situation a test of will and said the U.S.-led coalition will meet that test, even as the United States' allies began feeling pressure amid the increased violence.
U.S. forces had to rush reinforcements to help Bulgarian troops in Karbala, southwest of Baghdad. Polish troops also fought with members of al-Sadr's militia in that holy city. In Warsaw, Polish Prime Minister Leszek Miller promised that he would not abandon the coalition, despite growing pressure at home to withdraw from Iraq.
In the midst of this fighting, al-Sadr said Iraq will become "another Vietnam," as he put it, for the United States -- a growing insurgency that will gradually deplete the United State's military strength, and its will to fight.
This parallel is apt, according to Peter Kuznick, a professor of history at American University in Washington who specializes in military affairs. Kuznick told RFE/RL that in Vietnam, the U.S. military, with all its might, was no match for Vietnamese insurgents.
"If you've got a country that's united, or somewhat united, and has a vision of what it's fighting for -- as opposed to an occupying force confused about what it's fighting for -- you're in a very difficult position," Kuznick said.
Kuznick also pointed out that in the 1960s and '70s, the United States blamed the war in South Vietnam on what they called "invaders" from North Vietnam. In fact, he said, many of the enemies of the United States were South Vietnamese.
Today, Kuznick said, the Bush administration is blaming the violence in Iraq on a few former members of Saddam Hussein's government, and on foreign "terrorists." He said the recent fighting by both Sunni and Shi'a insurgents shows that the opposition is more broadly based and shared by groups that are normally hostile to each other.
"What the United States has managed to do is something that Saddam Hussein was never able to do -- unite the Iraqi people not for some kind of positive vision, but against the United States, against the U.S. occupation," Kuznick said.
Kuznick said he does not believe the Sunni and Shi'a militants have put aside their differences and become allies. Rather, he said, it appears to be more of what he calls "a marriage of convenience" based on common political ground -- opposition not only to the coalition forces, but also to the interim government they have set up.
Jack Spencer, who studies defense issues at the Heritage Foundation, a private policy-research center in Washington, conceded that U.S. and allied forces are facing the challenge of simultaneously fighting both Sunni and Shi'a insurgents, but he said he does not believe there is even a temporary alliance of convenience between the two groups.
Spencer said that "coincidence" would be too strong a word, but he said he believes the Sunnis and the Shi'a insurgents are fighting coalition forces now simply because both groups face the same deadline.
According to Spencer, those directing the fighting are eager to have Iraq ruled on their own terms, not by a democratic government. He noted that in less than three months, the U.S.-led Coalition Provisional Authority is scheduled to transfer sovereignty back to the Iraqis.
"I think that we have a deadline coming up on June 30, and those who would like to see democracy fail in Iraq are doing whatever they can to create obstacles to achieving that end, Spencer said. "Given that violence is their preferred means to achieve their political ends, this is their last chance to prevent democracy from coming to Iraq."
Spencer said the timing also could be attributed to the maturing of plans by the leaders of the insurrections. He said that, after the defeat and dispersal of Saddam's military one year ago, it took time for leaders of the resistance to mount a serious challenge to the occupation forces.
"It is possible that [the leaders of the current uprising have] been biding their time, coming up with a plan for a year, waiting for the optimal time to consolidate their forces and to put up one powerful fight, or two powerful fights, in a condensed area," Spencer said. "And that might be what we are seeing now."
Spencer said the leaders of the current Iraqi rebellion are probably local politicians and religious leaders, adding that it is unlikely they are remnants of Hussein's Republican Guard.