And that is a best-case scenario, as articulated by US Defense Secretary Robert Gates on Friday after Bush announced plans for more modest troop cuts by mid-July.

"One of the sad aspects of war is there is no script," Gates told reporters. "That history hasn't been written yet. And the enemy has a vote."

"I can tell you what my hope is. My hope is that the situation continues to develop, as it has, for the last several months, as we anticipate it will for the next several months, though the end of the year."

If it does, he said, US force levels would go down to about 100,000 troops, or from 20 combat brigades to 10, by the time a new president takes office in January 2009.

Bush and Gates sketched out hopes for a shrinking US military presence in Iraq as conditions improve, but no end point to a mission that has lasted four and a half years, claimed the lives of 3,773 US troops, and cost nearly half a trillion dollars.

Gates said it was crucial to reassure allies in the region and "signal potential adversaries that we are not leaving Iraq to their ambitions, and that we will remain the dominant force in the region."

Richard Haas, a former senior State Department official, in comments published this week by the Council on Foreign Relations said he could imagine "an American presence of say 75,000 troops for years, if the costs were not high."

"Again, I think the bottom line is that the administration has probably bought itself sixteen more months of something that looks a lot like the status quo."

In testimony and media interviews over the past week, Petraeus has refused to discuss his plans beyond the modest cuts that Bush announced.

But he has said conditions are stable enough in some areas that US troops can be pulled out, while keeping US combat forces in contested areas such as Baghdad and Diyala province.

A color-coded chart prepared for Petraeus's congressional hearings shows more than 10 US combat brigades currently leading operations, and another five brigades partnered with Iraqi forces.

By July, the numbers of US brigades in the lead or partnered with Iraqi forces fall to just over 10, as the mission of US forces begins to shifts to tactical, operational and ultimately, strategic oversight.

Petraeus said he intends further reductions of US troops but that the pace will depend on conditions on the ground.

A key unknown is whether security will hold after US troops leave.

A commission led by former marine corps commandant Jim Jones warned last week that the Iraqi military will not be ready to take over security for 12 to 18 months. The national police are so corrupt and sectarian they should be disbanded, the commission said.

Petraeus's answer is to "thicken" local defenses with neighborhood militias and "volunteers," creating a patchwork of stability, while national Iraqi forces are being strengthened.

The assassination Thursday of a leading sheikh in Al-Anbar province, a model for the new approach, underscored its fragility.

And in overwhelmingly Shiite southern Iraq, a violent struggle for power between rival factions has grown in intensity as British troops have pulled back.

Petraeus and his commanders also are seeing a big push by Iran to arm and train proxies among the Shiite factions, a significant development that analysts say adds a new level of risk to the situation.

"I think they want to keep the United States tied down in Iraq -- and painfully tied down -- for as long as possible on the assumption that that will reduce the risk of a ... US air strike against their nuclear facilities," said Toby Dodge, an expert at the International Institute for Strategic Studies and a former Petraeus adviser.

"Now the great irony about this is that Iranian policy in Iraq may be bringing forward, or making much more likely what it is designed to stop, which is a US attack on Tehran," he said.

Petraeus demurred this week when asked whether he should be given the authority to attack Iranian operatives inside Iran, saying it was outside his area of responsibility.

related report
Fewer foreign attackers entering Iraq: US general Petraeus
There has been a "significant" fall in the number of foreign fighters attacking US-led coalition forces in Iraq, the top US military commander in the country, General David Petraeus, said Friday.

"We have seen the reduction in the flow of foreign fighters that does appear to be statistically significant," Petraeus told Al-Hurra, a US-funded Arabic-language television station based outside Washington.

"Some source countries in response to diplomatic overtures have taken action to make it tougher for foreign fighters, would be suicide bombers to travel through Syria into Iraq," he said.

"We know that Saudi Arabia has made it tougher for military age males to travel on a one way ticket to Damascus airport ... We know that Libya has interdicted the flow of some of these individuals."

President George W. Bush has long accused Syria of letting fighters pass from its soil into Iraq to launch attacks against US forces, and Iran of backing such insurgents.

"We think that Syria has taken some actions" to reduce the influx of foreign fighters, Petraeus said, but added it was "difficult to say. We don't have direct conversations with them on that subject."

Petraeus gave a key report to Congress this week on progress of the "surge" strategy which saw thousands of extra US troops arrive to bolster security in Baghdad and the volatile western province of Anbar.

He said the strategy was managing to quell some of the rampant insurgent violence that has threatened to tear Iraq apart under the US occupation, and proposed to end the surge and begin reducing the troop deployment next year.