3 Commando Brigade Royal Marines is the elite fighting force of the Royal Navy.
Marines have to undergo rigorous training
The 3,500-strong brigade is an amphibious rapid reaction force, highly trained for combat in extreme weather conditions and on difficult terrain.
The frontline of the brigade is made up of three lightly armed units - 40, 42 and 45 Commando - which each have about 700 members.
It maintains a state of permanent readiness to move anywhere in the world when the security of Britain and its allies is under threat, and can be deployed early when needed to indicate the UK's political and military will and capability.
Royal Marines spokesman Major Tim Cook told BBC News Online one of the most important aspects of the brigade was that it could be "poised" for action off a coast in international waters.
"They can sit, sail up and down the coast and exert pressure - they provide the pressure to back up the diplomacy."
3 Commando Brigade was formed during World War II, and played a prominent role in the D-Day landings.
It led the final assault on Port Stanley in the Falklands campaign of 1982.
3 Commando Brigade
Formed during World War II
Part of the UK's Joint Rapid Reaction Force
Includes three lightly armed units - 40, 42 and 45 Commando
Undergoes rigorous training in extreme weather conditions
It has also been deployed in Northern Ireland, Sierra Leone, and on peacekeeping missions in Bosnia, the Congo and Kosovo.
Most recently, 40 and 45 Commando have served in Afghanistan as part of the war on terror.
It is 40 and 42 Commando which are being sent to the Gulf as part of the largest British naval deployment for two decades, led by HMS Ark Royal.
Commando soldiers have to undergo what is recognised as one of the toughest infantry training regimes in the world, at the Commando Training Centre in Lympstone, Devon.
Much of the training is carried out on the rugged terrain of Dartmoor, and culminates in the Commando course - a series of tests of fitness, endurance and military professionalism.
While reluctant to sound to self-congratulatory on behalf of the Marines, Major Cook admitted the course was "tough", adding that when compared with infantry training around the world "very few other courses could stand up to it".
Royal Marines who pass the course win the prized green beret, and then normally join a Commando unit of 3 Commando Brigade, led by Brigadier James Dutton.
40 Commando is based near Taunton in Somerset, 42 Commando near Plymouth in Devon - where the brigade headquarters are also located - and 45 Commando at Arbroath on the east coast of Scotland.
Norway is one of the locations used for training
The units undergo rigorous environmental training in either "hot, arid", "humid jungle" or "cold mountainous" conditions.
Usual training areas are in the Middle East, Belize or Brunei, and Scotland and Norway.
As well as frontline Commando units, the brigade includes combat support elements provided by the Army.
These include 29 Commando Regiment Royal Artillery, and 59 Independent Commando Squadron Royal Engineers, which provide the specialist artillery and engineer support for the brigade.
Landing craft, such as hovercraft and raiding craft, are provided and operated by 539 Squadron Royal Marines
The brigade also receives vital support from Commando Logistic Regiment Royal Marines, which provides services ranging from repair and recovery of equipment to food supplies and medical treatment.
The brigade has its own reconnaissance force in the form of the Brigade Patrol Troop, a small but powerful force which provides early warning and intelligence information.
It usually operates up to 60 kilometres ahead of the main brigade.