As US troops battled uprisings in Sunni and Shiite sections of Iraq, many experts now believe that the US operation in Iraq is facing its biggest risk since the fall of Baghdad.
But there are increasing political divisions in the United States over the way forward. The June 30 handover date to an interim government and the number of troops in Iraq have all become subjects of debate.
Experts are agreed however that the United States has to be firm without heightening opposition to its occupation plan.
"The US has little choice but to suppress militias and insurgents, otherwise stabilising the country will become impossible," said Loren Thompson, an analyst at the conservative Lexington Institute defence research centre.
But he added that US forces had to avoid tactics that could inflame the majority Shiite Muslim community, who have until recently largely favoured US attempts to restore order after the fall of Saddam Hussein's government.
"The recent setbacks in Fallujah and Sadr city do not reflect popular opinion which is mostly passive but supportive," said Thompson.
Lawrence Korb, a deputy defence secretary in the 1980s administration of Ronald Reagan, also said that after losing the support of many Sunnis, the United States cannot afford to lose Shiites, who have been called to action against US forces by the radical cleric Moqtada Sadr.
"We've lost a lot of Sunnis to begin with, you don't want the Shiites against you too," Korb told AFP.
Korb said that the closure of Sadr's daily newspaper for inciting violence had been a mistake. Sadr has now been declared an "outlaw" and is being protected by his militia. US forces should not have escalated the confrontation with Sadr and his followers, the expert added.
US officials in Baghdad and Washington have publicly sought to minimise the importance of the Sadr uprising. But privately many now acknowledge that the US campaign in Iraq is in its most decisive phase since the fall of Baghdad one year ago.
Top US officers have concentrated up to now on carefully targeted raids. A senior US Central Command official said, on condition of anonymity, that there would no question of detaining Sadr "after a sermon or entering a mosque".
The military has sought to avoid the use of its most sophisticated weapons.
But faced with an escalation in violence, two 500-pound (227 kilo) precision-guided bombs were used Wednesday against a mosque compound in the city of Fallujah where insurgents were believed to be hiding, according to Brigadier General Mark Kimmitt.
The Defense Department is also preparing plans for a possible increase in troop numbers in Iraq. General John Abizaid, the head of Central Command, this week asked his top officers to draw up options for possible reinforcements.
The increased violence has highlighted the weakness of Iraqi police and security forces, who the US occupying force admits are ill-equipped and not well enough trained to cope with their tasks.
"We wouldn't have put police on the streets of US cities with so little training and equipment," said Korb.
"We're paying for bungling," he said, criticising the small number of troops sent to Iraq at the start of the invasion, the lack of guidance for events after the war, and the failure to break up militias when the US administration had ordered the disbanding of the Iraqi army and police.