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I'm home after observing at very close quarters the US marines push to re-take the town of Now Zad in Helmand Province.

It was an intense and violent experience.

You can see for yourself below how it went, but also the questions that a major initiative like this leaves unanswered.

One thing is clear now the dust has settled. The Afghan timescale can rarely keep pace with the American one - particularly now President Barack Obama has added such a sense of urgency to his Afghan surge.

Only a small proportion of the security forces in the Now Zad area are Afghan, and attempts to get a district governor's office up and running in the wake of the operation are going more slowly than planned.

NOW ZAD, HELMAND PROVINCE - For several days past we have accompanied US marines as they fought their way through this town.

The exact sequence of events and story of our experiences will be the subject of an extended special report on Newsnight in the future.

However in outline what happened was that 1,000 US marines with British, Afghan, and Danish support, fought to clear insurgents from what was once one of this province's thriving district centres, but which has for the past three years been abandoned by its 20,000 plus inhabitants.

It was a very large set piece operation that played to all of the strengths that the US marines could bring - heliborne landings, air cover; massed drones for surveillance, tanks, and an extraordinary level of engineer support for troops who had to assault through an "IED belt" of hundreds of devices laid by the insurgents.

I wonder at the end of it all though if the local insurgents had not also shown what they were capable of.

The operation's commanders were hopeful of cutting off the insurgents' lines of withdrawal, pinning them in place, and fighting them.

Certainly the coalition troops succeeded in carrying out their complex tasks without loss on their own side.

They found a large number of IEDs and other weapons in caches uncovered during house to house clearances.

They also detained a number of individuals they had been looking for.

The insurgents however, did what any good guerrilla force will do when confronted with such a show of strength - they melted away.

In most places they were not willing to engage the marines on the terms of their own choosing.

Furthermore within a day of the operation starting, the Taliban were trying to get used to the re-drawn battle lines and start setting new IEDs.

On the second day of the operation, marines discovered two Afghans blown up by their own bomb as they tried to set it on one the lanes cleared the day before.

The terms of the fight in Now Zad have been changed dramatically by the events of the past week.

But the question of what exactly this large set piece operation achieved is one requires a longer term view.

It is time now for us to start the journey back home with all our material.

Certainly what we saw in Now Zad heralds a new phase of this campaign, as US forces start their new build up.

In many ways what we have seen may be the prototype for wider operations aimed at clearing Taliban strongholds.

NOW ZAD, HELMAND - US marines have launched a major operation designed to clear a key part of this province of insurgents.

It is the first major demonstration after President Barack Obama's speech of US determination to take the initiative from the Taliban and in the words of one officer here, "crush them".

Now Zad was a thriving market town of around 20,000 inhabitants until British troops came here in 2006.

During the battles that followed a school was bombed by Nato aircraft and the entire civilian population left, creating a ghost town.

The aim of the operation which began on Friday is to clear the town before bringing back the Afghan district governor.

US marines have poured into the area during recent days and during the early hours of Friday morning staged heliborne landings designed to block either the withdrawal of those fighters in the town or their reinforcement by others in the surrounding area.

During the past two years the Taliban are thought to have sewn thousands of Improvised Explosive Devices in the abandoned town.

So this morning's air landings were followed by a breaching operation by combat engineers.

They have been firing Mic-Lics - rockets which tow a hose full of high explosive behind, to clear lanes through areas of suspected IEDs.

The hose contains 1,700lbs of high explosive which detonates, destroying any buried mines or devices, and creating a path for troops to follow.

These operations are seen both as a chance to end the two year stalemate in Now Zad, something which one US commander has described as an "un-marine-like strategy" and to trial techniques for entering other guerrilla strongholds in this and Kandahar provinces.

Soldiers from the 3rd Battalion, 4th Marine Regiment, with which we are embedded, held a prayer service before moving into the operation this morning.

Estimates of guerrilla fighters in Now Zad and its surrounding area range from 2000-3,000.

Nobody really knows whether the Taliban will fight the marines as they move through the town clearing it. That is, as one lieutenant here put it, "the billion dollar question".

Classic guerrilla theory would suggest that they should avoid confronting the large forces massed for this operation.

But the loss of the town would be a symbolic blow for the insurgency, so a hard fight is expected at some stage.

The US assumed responsibility for Now Zad from the British last year. Since then thousands of American soldiers have been sent to Helmand, and now, many thousands more are on their way.

These extra numbers have allowed them to mass forces for offensive operations like this one.

If these additional troops are not simply to be sunk into the defence of newly conquered districts in the backwoods of these southern provinces, Afghan forces will have to be increased to take over security in places like Now Zad.

Currently the Afghan army has a nominal strength of over 7,000 in Helmand but an actual presence of about half that, since so many are absent for a variety of reasons.

Afghan security forces are already spread thin, and their presence on this operation is very small compared to that of the Americans.

The risk then, as the US steps up its clearing operations is that Mr Obama's surge will get ahead of anything the Afghans can manage.

WITH US MARINES IN HELMAND - During the invasion of Iraq David Petraeus, then a US major general commanding the 101st Airborne Division, turned to a reporter and said: "Tell me how this ends."

Americans are much happier when they know the narrative arc, and this is what President Barack Obama has tried to provide for Afghanistan.

Fate decreed that Gen Petraeus would be the man who in 2007 directed not a happy ending in Iraq - far too many lives were lost for it to be called that - but an end point for the US military presence after they got on top of the insurgency, leading to a dramatic fall in violence.

By decreeing that his forces will surge before starting to drawdown in Afghanistan in the summer of 2011, the president has tried to tell us how this Afghan war will end.

So it is one of those ironies of politics that in his speech at West Point Military Academy on Tuesday, Mr Obama once again blamed the Bush administration for taking its eye off the Afghan ball.

But he is attempting to copy his predecessor's strategy in having a "surge" that will bring matters to a head, then a positive outcome.

Clearly the White House hopes that providing the 2011 start point for beginning a drawdown will simultaneously concentrate Afghan President Hamid Karzai's mind about the need to improve governance while fighting corruption, give US troops in the field clear direction and offer some hope that the repeated deployments required to sustain high troop levels in both Iraq and Afghanistan during the next year will eventually ease off.

It has been apparent for some time that Mr Obama would go for this strategy. Indeed back in mid-October I reported that he had resolved on a large increase in forces that could exceed 45,000.

My story was swiftly rubbished by the White House press secretary. One or two parameters did change - principally the reduction by one combat brigade of General Stanley McChrystal's troop request - but even so, people are now talking about total Nato forces rising from 90,000 now to 138,000 late next year.

If the basic shape of the announcement was clear to the president back in mid-October (and was communicated to the British government, which is how I learned of it) why did he wait all these weeks to announce it?

It would seem that the untidy results of the Afghan election and the need to reassure doubters in his party caused the delay.

Ballot rigging and expressions of dissent at home threatened the clear narrative that Mr Obama wanted to set out and has now finally done.

Certainly US commanders here seem to welcome the idea of boosting their forces and going all out to suppress the Taliban in the coming months.

They are naturally aggressive, and believe they can dominate this country, rather than allowing it to dominate them.

The lessons of this year's fighting do not yet tell us whether the American faith in boosting their forces is justified.

This summer troop numbers have gone up and so have violent attacks on the coalition - and therefore casualties.

US commanders, including Gen Petraeus himself, told us to expect heavier casualties this year, because there would be more troops in the field taking risks.

In Iraq, surging troops did work - when combined with other measures such as turning the tribes and intensive special operations - because it was able to bring security, to make Iraqis in certain key places feel safer.

Will surging here achieve the same effect or simply make more enemies?

The other key issue that must be resolved in Afghanistan is whether the Afghan security forces can be increased fast enough to consolidate the gains made in clearing operations.

Commanders from Gen McChrystal downwards are acutely aware of this and at least 5,000 additional troops are to be assigned to training those local troops.

It will not be just about numbers either - these Afghan forces will need to be of the right quality.

If not a different Iraqi model may be repeated, that of 2004-5 when coalition troops swept neighbourhoods, suffered casualties and then simply saw the insurgents return or poor Iraqi troops melt away when operations were over.

This is the real risk of Mr Obama's approach - that it attempts to force the reality of Afghanistan's insurgency into an American narrative, with its own plotline and duration.

There is no doubting the determination of troops here though to try to curb this historically unruly place to their will.

The enemy though "gets a vote" and will do his best to thwart them.

Over the coming days and weeks, intensive operation will be launched, particularly in the south of the country.

That is why I am now in the field with US troops and will send updates soon.