The United States military's special operations units are expected to lead the fight in "America's New War."

They are the elite -- composed of the Navy SEALs, Army Rangers, Marine Expeditionary Units, Delta Force and the Air Force Special Operations Forces.

Britain's SAS is one of the world's best known and most respected elite force.

Click on below to see the troops involved and what they do.

ARMY RANGERS

As with all special operations forces, the U.S. Army Rangers report to the Joint Special Operations Command (JSOC) when in hostile or war situations. In addition to the Rangers, which consists of one active regiment with three battalions, the U.S. Army Special Operations Command includes Army National guard, civil affairs, psychological operations and chemical reconnaissance groups.

But the Rangers, themselves, are the spearhead of the Army's special operations forces. Ready to deploy by land, air and sea anywhere in the world at a moment's notice, Rangers specialize in rapid infantry assault, night fighting and airfield seizure.

According to the JSOC, the Rangers have taken part in every major combat operation since the end of the Vietnam War. In many cases, they have are supported by the 160th Special Operations Aviations Regiment -- better known as the "Night Stalkers" -- which use state-of-the-art aircraft and equipment to assist all special operations forces from the air.

GREEN BERETS

U.S. Army Special Forces (Airborne) a.k.a. The Green Berets

The Green Berets are used both as a combat force and to train guerrilla troops in other countries. Each member must be able to speak at least one language other than English.

The Green Berets are used for a variety of missions such as unconventional warfare, special reconnaissance, direct action and counter terrorism.

The Special Forces (Airborne) served in Vietnam, Operation Just Cause (in Panama) and Operations Desert Shield and Desert Storm.

MARINES

Marine Expeditionary Units

While not technically a "special operations force," the Marines advertise their Expeditionary Units as "special operations capable." These units -- at least two of which are in the Indian Ocean -- are capable of quick, compact, multi-faceted military campaigns.

These units generally have more equipment, logistical and technical support, weaponry and marines than their conventional counterparts, giving them more firepower. Their purpose is to provide the commander-in-chief an operational maneuver capability from the sea.

Commanded by a colonel, one Marine Expeditionary Unit typically includes about 2,200 personnel. The infantry battalion uses amphibious assault and light-armored vehicles, and the aviation unit employs helicopters and fixed-wing aircraft such as "Harrier" jets.

NAVY SEALS

SEALs, which stand for Sea, Air, Land, are the U.S. Navy's primary and most acclaimed special operations forces unit. Operating mainly in tight 16-man groups, SEALs are capable of conducting top-secret ground and water-based missions.

SEALs are trained extensively and rigorously in and around San Diego, California, and Norfolk, Virginia, to withstand and, in fact, thrive in the face of personal, physical, environmental and other challenges. They trace their history to the frogmen of World War II, although new threats -- including terrorism -- have forced them to evolve substantially in recent years.

SEALs take on missions, many of them classified, throughout the world. Larger teams are often divided into "cells" -- i.e. an evasion and recovery cell, force protection cell, sniper cell, etc.

DELTA FORCE

While the military acknowledges the existence of Special Mission Units, or SMUs, such as Delta Force, it does not specify what they are called or their specific locations, staffing or organizational structure. Unofficially, Delta Force is considered one of the U.S. military's elite special operations forces units, its members drawn from all military branches.

Delta Force is thought to have been formed after a spate of terrorist attacks in the 1970s. As with the Navy Seals, several allied special operations forces -- including possibly those of Great Britain, France, Germany and Israel -- have significantly influenced Delta Force.

The composition, strength and abilities of Delta Force remain a closely guarded secret, as are all of its missions to date. But the unit is thought to be equipped with the nation's most advanced weaponry and equipment. Unit members undertake an extensive selection and assessment process, and conduct the core of their counter-terrorism training on U.S. military bases.

AIR FORCE

Air Force Special Operations Forces

While not as widely known as the SEALs, Army Rangers or even the mysterious Delta Force, the Air Force Special Operations Forces transports and provides close-air support as an integral part of all any special operations mission.

Air Force SOF are uniquely qualified to provide close-air support, firing on targets and participating in refueling, re-supplying and other support tasks. The units utilize advanced navigational and surveillance equipment.

The Air Force SOF -- in fact, all the U.S. military special operations forces units -- pride themselves on their "quiet professionalism." Besides physical and military challenges, special operations forces are also schooled in language, cultural and social differences in their assigned areas of responsibility all over the globe.

The Britan's SAS -- an elite force

The SAS -- Special Air Service -- is one of the world's best known and most respected elite forces.