(Edits)

By Ross Colvin

BAGHDAD, Sept 5 (Reuters) - A photograph can mislead because it freezes a moment in time and can't show what happens next, an issue U.S. lawmakers will face when General David Petraeus gives them his "snapshot" of Iraq next week.

The top U.S. commander in Iraq has promised a "forthright snapshot" of the situation on the ground when he testifies before U.S. Congress on Sept. 10 on the build-up of 30,000 more U.S. troops since February and military progress in Iraq.

Petraeus's assessment is likely to contain few surprises. He is certain to say his counter-insurgency strategy has succeeded in reducing violence between Shi'ite and Sunni Arabs that has killed tens of thousands of people.

But what happens next? With opposition Democrats and some Republicans calling for U.S. troops to be brought home, it is Petraeus's recommendations that will receive the most attention.

He suggested this week that he may recommend a troop reduction in March to avoid overstretching U.S. forces. Making an unannounced visit to Iraq on Monday, President George W. Bush also raised the possibility of troop cuts, but said any reduction must be "from a position of strength".

A number of the lawmakers who will be listening to Petraeus have also visited Iraq in recent months to see for themselves that the violence has indeed ebbed, although hundreds of people still die in suicide car bombings and shootings every month.

The figures Petraeus selects to quantify the fall in violence will probably be hotly debated. Iraq's government has banned ministries from publicly issuing death toll figures, viewing them as politically sensitive and open to manipulation.

Lieutenant-General Raymond Odierno, the number two U.S. commander in Iraq, said on Tuesday that last week had seen the lowest number of violent incidents against civilians and security forces across Iraq in the past 15 months.

Baghdad's residents have certainly seen a sharp decline in the number of daily bombings, mortar attacks and shootings. Sectarian death squad killings have been halved -- from 40-50 a day to less than 20, according to Iraqi police figures.

But the data on the violence is complex.

For example, figures compiled by the Brookings Institution, which tracks "multiple fatality bombings" in Iraq, show a 50 percent drop in such attacks between February and August, but the death toll has fallen only slightly.

Data obtained from Iraqi ministries show 1,773 civilians died in Iraq in August, a nearly 8 percent rise over February.

SUNNI REBELLION

With some fearing the crackdown is just delaying an inevitable battle for power both between and within Iraq's sectarian and ethnic groups, a pressing question is how much longer the U.S. military should or can sustain the "surge".

It has already exacted a bloody toll -- more than 600 U.S. soldiers have died since February. U.S. commanders have spoken of starting to reduce troop levels in the first half of 2008.

The U.S. strategy has benefited hugely from a rebellion by Sunni Arab tribal leaders against al Qaeda. In his report, Petraeus will likely highlight the success in pacifying restive western Anbar province, once considered lost to Sunni Arab insurgents and the most dangerous place in Iraq for U.S. troops.

But senior U.S. military officers warn that unless the Shi'ite-led government does more to bring the tribes into the fold, they could soon be fighting U.S and Iraqi soldiers again.

"The drop in violence (in Anbar) is tied largely to cooperation with the U.S. The same fighters that were killing Americans could be killing them again in a matter of weeks or months if the central government does not act," said Anthony Cordesman, a prominent Washington-based Iraq expert.

Petraeus says the aim now is to hold all the military gains that have been made. But it remains to be seen whether Iraq's security forces can hold cleared areas on their own.

"Increasingly, the Iraqi army is a viable partner for the U.S. when they go on patrols together, but I don't think that it is in a position to handle most of the regions on its own," said Michael O'Hanlon, an analyst at the Brookings Institution.

Petraeus will also say that U.S. forces have severely disrupted al Qaeda in Iraq. But al Qaeda has proved resilient in the past and U.S. commanders stress that the war will not be won on the battlefield but by fostering national reconciliation.

"We certainly have seen modest gains in the overall security environment. Those gains are likely to be ephemeral ... because the political atmosphere is still so contentious," said Ted Carpenter, an analyst at the Cato Institute think-tank.

(Additional reporting by Andrew Gray and Kristin Roberts in Washington)

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