By Paul Adams
BBC Chief Diplomatic correspondent
Britain is sending 850 troops from the Black Watch regiment and supporting units from the south of the country to a position close to the Iraqi capital - a request that has led to protests from soldiers' families and a number of UK MPs.
Most British soldiers are currently based in southern Iraq
Geoff Hoon finally addressed this on Thursday, in his statement to MPs.
Critics were wondering why this deployment was necessary when the Americans have more than 135,000 troops in Iraq.
But Mr Hoon said that "fewer than a third of US forces in Iraq have the requisite combat capability, and of those even fewer have the armoured capability that is needed."
In the words of military sources, the American "teeth to tail ratio" - the balance between fighting forces and supporting units - is different from their British counterparts.
We might assume the Americans have endless supplies of heavily armoured, battle-hardened troops, but they don't. What they have is already heavily committed.
Hence a request for British forces to lend a hand.
Why the Black Watch?
They are the divisional reserve, somewhat under-used in recent weeks.
Things have been very quiet in the south since the end of August. Commanders emphasise that we should see things in terms of coalition operations, not countries working individually.
And so it makes perfect sense for an under-used, hugely capable unit to be moved from one area to another.
The Black Watch is on its second tour in Iraq
The Chief of Defence Staff, General Sir Michael Walker, today emphasised that all coalition forces in Iraq are under overall American control, and it's perfectly natural for a corps commander to deploy his forces as he sees fit.
But defence sources admit that as this is the first time since the war that British troops will have come under American tactical control in this way, this is an "unusual" operation.
We have been asked to do similar things in the past, and have so far said no, for a variety of reasons.
One such example was a US request for British troops in Najaf in the summer - one the British felt unable to fulfil (it would have been a much larger deployment than this).
The military top brass think that the time has come to say "yes" and they believe that the circumstances (the nature of the location in question, the availability of the Black Watch, the situation on the ground in the south) make this feasible.
They feel, in the words of one very senior British officer, that this is about "being a good ally."
Whose command will the troops be under?
The British troops will be under the tactical control of the US marines, the 24th Marine Expeditionary Unit.
Agreement has already been reached on the mission and tasks and if the US marine commander wants to change these he would have to refer back to General Rollo, the British commander in Basra, who remains in overall operational command.
The commanding officer of the Black Watch will determine how those tasks are performed within his geographical area of operations.
Will we leave the south exposed?
The removal of an armoured battle group does pose a certain risk, but Geoff Hoon told MPs the level of risk was "militarily acceptable."
The divisional reserve role will be taken by the 1st Battalion, the Scots Guards, with their own Warrior armoured vehicles.
Why did British troops only check the area yesterday?
It's not particularly surprising that it's taken place at this relatively late stage.
The British and Americans will have been talking about this idea for some time (possibly weeks), with plenty of intelligence being exchanged.
Why were there so many rumours prior to the announcement?
There was a briefing to Black Watch families, in the UK, about 10 days ago.
The possibility of a deployment was raised.
This gradually filtered out through the Scottish press and a process of Chinese whispers resulted in the "Black Watch to Falluja" rumour.
MPs finally heard about it and wondered what was going on.