The battle for Falluja is supposed to be the beginning of the end.
By Paul Reynolds
World Affairs correspondent, BBC News website
The plan is that US troops, supported by forces from the interim Iraqi government, will drive a stake through the heart of the insurgency, thereby opening the way for elections to take place across the country on 27 January next year.
US troops begin assault on Falluja
In turn, those elections are designed to produce a transitional assembly and government, leading to a new constitution and a fully elected government by the end of next year, after which the foreign troops can think of leaving.
Is this therefore a decisive moment?
The problem with "decisive moments" is that they tend not to be so and to be followed by other decisive moments.
The battle for Falluja is one that the Americans and the interim Iraqi government have to fight if they are to impose their will on the country.
It is, however, at best a stage in the pacification of Iraq and not the solution itself.
It incidentally shows that Saddam Hussein's prediction that the real war would begin only after the invasion had some truth in it.
A parallel is being drawn in some quarters with the British Army's Operation Motorman in 1972 in which "no-go areas" held by the Irish Republican Army were retaken. That, too, was declared as a decisive moment.
If it was, it took a long time to take effect. The IRA decided to melt away and fight another day. And the fight lasted another 30 years or so.
Another parallel more relevant perhaps for the US marines is the battle they fought to regain control of the South Vietnamese city of Hue after the Vietcong's Tet offensive in January 1968.
That battle was seen at the time as instrumental in bolstering the South Vietnamese government but in the long run the Vietcong and the North Vietnamese won anyway.
The marines are hoping that this time the final outcome will be different - that the insurgents will prove to have no real strength in depth, no easy source of supply and that they can be beaten.
The rebels in Falluja, or at least some of them, appear to have decided to fight.
Role of Iraqi forces
The first hurdle therefore that this operation has to overcome is to avoid major civilian casualties.
If it turns out to be a blood bath, the warning from the UN Secretary General Kofi Annan that it might jeopardise the very political process it is designed to support, could come true.
It is important for the interim Iraqi government to be seen as the leader, even though the main fighting will be undertaken by the Americans.
It will be tempting for the Americans, for political reasons, to play up the role of the Iraqi troops.
US marines prepare for attack
Already there have been television pictures of Iraqis in smart uniforms. The issue is whether the Iraqi forces play a more substantial role than that of video extras.
The interim Prime Minister Iyad Allawi has certainly presented himself as the man in charge.
But if he does not finish the job as he wants, he will probably be finished himself.
A sign that the British Foreign Office wants to play its part in boosting Mr Allawi came when it placed transcripts of his statements on its own website.
After the battle
Key problems will remain after the battle is over.
The hope among US and Iraqi government strategists is that the insurgents will at last be on the run and that the tide will be turned.
The fear is that the insurgents will gain more support, that they will be dispersed but will not disappear.