In any case, the exact level of US troop reinforcements is less than clear. There is apparently some confusion as to whether the 21,500 additional combat troops being sent will be accompanied by the even larger number of support troops that usually back up the combat brigades. The current overstretch in the US army means that existing support troops may have to carry much of the extra work, and this creates something of a dilemma: either additional support is forthcoming (which could mean another 30,000 troops), or else it will be difficult to maintain the new level of forces for any length of time.

Whatever happens in Iraq, a significant build-up of US forces in the Persian Gulf appear to have much more to do with a possible crisis with Iran. The previous column in this series pointed to four important recent military decisions:

• the order to US forces in Iraq to kill or detain members of Iran's Revolutionary Guards who might be operating in Iraq

• the decision to deploy a second aircraft-carrier battle-group to the region for the first time in nearly four years

• an increase in the number of Patriot anti-missile batteries

• the assignment of additional mine counter-measures ships (see "The United States and Iran: the logic of war", 1 February 2007).

In addition, two unusual and lesser-noted developments deserve attention, for they could carry even greater implications for the future course of events.

A naval surprise

The first concerns the USS Ronald Reagan, an aircraft-carrier that is at the centre of yet another carrier battle-group that may be heading to the region. The Ronald Reagan left San Diego on 27 January at the head of a powerful force that included the guided missile-cruiser USS Lake Champlain and the guided missile-destroyer USS Russell; both warships are reported to be equipped with scores of sea-launched cruise missiles.

The US navy reports that the Ronald Reagan carrier strike group "was deployed under the navy's Fleet Response Plan (FRP), which provides the United States with the ability to respond to any global commitment with flexible and sustainable forces and the ability to respond to a range of situations at short notice."

Two aspects of the carrier's operations are of interest. The first is that the carrier only returned from its previous deployment to the Arabian Sea in July 2006, something usually followed by a longer period of home-porting; instead, the ship has since been taking part in exercises designed to maintain a high state of military readiness. The second concerns the current operations. The navy says that the carrier is actually deploying to the western Pacific to take over from the older carrier, the USS Kitty Hawk - this being the only carrier permanently based abroad (at Yokosuka in Japan), and currently undergoing maintenance.

What is a bit unusual here is that the Kitty Hawk's three-month maintenance work is being undertaken by the crew, not external contractors, with the ship retaining the ability to deploy at short notice if need be. Furthermore, this maintenance started on 8 January, whereas the Ronald Reagan carrier battle-group will not even arrive in the area until mid-February. This does not seem to fit with standing in for the Kitty Hawk and does mean that another carrier battle-group could actually be deployed to the Gulf at short notice. Moreover, this is a strike group that has a crew experienced in recent operations in the Gulf.

A third carrier battle-group deployed to the region would be a clear sign that, at the very least, a major show of forces is in prospect; but in another sense, and from a rather different angle, that is beginning to happen already.

In addition to maintaining an aircraft carrier battle-group in the Persian Gulf region since the Iraq war began in 2003, the US navy has also tended to keep another type of naval force there as well. This is termed an "expeditionary strike group" (ESG): it comprises of a very large amphibious warfare ship, normally a Wasp-class 45,000 ton warship, accompanied usually by two smaller amphibious warfare ships - although even these are as large as the Royal Navy's two Albion-class vessels.

The current ESG in the Gulf is centred on the USS Boxer, together with the USS Dubuque and the USS Comstock, as well as a cruiser, a destroyer and other supporting vessels. This flotilla alone has well over 2,000 marines on board, equipped with helicopters, AV8B strike aircraft, landing craft, tanks, armoured vehicles, an array of logistics support and even military hovercraft. It is designed to be highly versatile but is particularly suited to coastal and near-coastal operations against defended positions. The Boxer ESG has been in the area since late October and, in the normal way of things, might have another two months to go before being replaced by another group.

An accidental war?

This is the second major recent development noted above. For what has not been widely noticed is that a second expeditionary strike group - based on a sister ship to the Boxer, the USS Bataan - transited the Suez canal into the Red Sea on 30 January and has joined the US navy's fifth fleet, responsible for US naval operations in the Persian Gulf, Arabian Sea and surrounding waters.

Furthermore, the Bataan ESG is a particularly powerful flotilla; it includes two more amphibious warfare ships, USS Shreveport and USS Oak Hill, the guided missile-cruiser USS Vella Gulf, the guided missile-destroyer USS Nitze, the frigate USS Underwood and the nuclear-powered attack submarine USS Scranton.

Thus the naval forces now gathering in the region include two carrier battle-groups and two expeditionary strike groups, with the possibility of yet another carrier battle-group, centred on the USS Ronald Reagan, being barely ten days away.

This combination of events does not mean that a war with Iran is imminent, although it does mean that a remarkable concentration of forces will be available to the US by late March and early April - including precisely those kinds of forces that would be employed to defend US and coalition assets in the region should a crisis with Iran escalate to open conflict.

That could happen in three ways:

• an Israeli attack on Iranian nuclear facilities, leading to Iranian retaliation against US forces

• a US attack on Iranian nuclear facilities, with a similar response

• a determined US show of military force to add to the diplomatic pressures on Iran that "goes wrong".

In other words, quite untoward and unplanned developments, even accidents, could combine to turn a tense crisis into conflict.

This military build-up, in the context of hawkish rhetoric from neo-conservative quarters in Washington, can be held to imply a firm intention on the part of the United States (or Israel) to launch an attack on Iran. As last week's column pointed out, this may not be the case.

What is actually more risky in current circumstances is a crisis getting out of hand, particularly in the complex and volatile circumstances that now prevail in Iraq as the American surge continues, as well as in the context of the uncertainties and infighting of Iranian politics.

From a strictly diplomatic point of view, this is a dangerous time for the United States to be sending powerful naval forces into the region, especially as there will be far greater media coverage of these moves in Iran than will appear in the western press. The powers that be in Washington think differently, however, and are making decisions accordingly. The effect could be to make April a rather dangerous month.