The official hymn of the US Marine Corps opens with the lines: "From the Halls of Montezuma to the shores of Tripoli" - an indication of the Corps' far-flung role around the world.

Now land-locked Afghanistan can be added to the list of countries where these sea-soldiers have fought.

The Marines owe their origin to units active at the time of the American Revolution.

But it was World War II that prompted a massive growth in the Corps, as Marine Units spearheaded the island-hopping campaign to wrest control of the Pacific from the Japanese.

Today the Marine Corps is effectively a small army and air force in itself. Unlike many other countries' marines, it is a separate service standing alongside the US Navy, Army and Air Force.

Its chief mission is to establish and secure footholds in enemy territory.

Spearhead force

Striking from specially designed amphibious warfare ships, Marines can either be put ashore from purpose-built landing craft and amphibious armoured vehicles, or, as in Afghanistan, they can be inserted by helicopter.

It was elements of the 15th Marine Expeditionary Unit that spearheaded the operation south of Kandahar, flown in on-board giant, twin-engined Sea Knight helicopters from carriers in the Arabian Sea.

A Marine Expeditionary Unit or MEU is the basic building block for any Marine force.

It has a strength of about 2,200 personnel and usually comprises a reinforced battalion of Marines, a composite aircraft squadron and supporting units.

The ground combat element - essentially an infantry battalion - is supported by artillery and light armoured vehicles.

It can draw on a variety of helicopters including the Super Stallion CH-53E and the CH-46E Sea Knight for transport and Super Cobra attack helicopters.

The beauty of the MEU concept is that this package is all embarked on board an Amphibious Ready Group of three or four specialised ships.

The US had positioned two MEU's off the coast of Pakistan for just such a mission.

'Mythic status'

From the country that invented fast food, this is if you like the military equivalent - a self-contained army and air force, ready-packed, ready-to-go.

Based largely on their World War II role, the US Marine Corps has a prominent place in the popular imagination.

Countless John Wayne films have burnished the almost mythic status of the Corps, who revel in their image of macho toughness and the idea that "the Marines lead the way".

The US Marines are undoubtedly among America's best light infantry forces and they appear tailor-made for this mission.

But the Marine Corps is, perhaps surprisingly, not just characterised by its brawn. It is in fact one of the more innovative and thinking of the armed services in the United States.

Not least, this is because if the country already has an army, navy, and air force, the US Marine Corps needs to justify its existence in some special way.

New techniques

Thus the Marine Corps has been in the forefront of developing new weapons and tactics to operate in urban environments.

And it is spearheading the fight against terrorism in another way as well.

A few years ago the Marines established new rapid response teams to deal with chemical or biological weapons attacks in the United States itself or against US targets abroad.

This task has been expanded to a broader role as a crisis response team against terrorist incidents.

And whatever their role abroad, the Marines look set to play a prominent part in the developing military aspects of homeland defence in the United States itself.