Echelons above corps in US and NATO practice, refer to higher headquarters, of purpose-built organization, which involve a greater number of troops than would be in an army corps. They may be standing organizations with a regional responsibility, or may be established for a particular operational purpose. While EAC most commonly refer to ground combat forces, they may refer to joint commands. They may also be administrative headquarters with responsibility for preparing combat forces.

While there were a significant number of EAC in World War II

World War II

World War II, or the Second World War , was a global conflict lasting from 1939 to 1945, involving most of the world's nations—including all of the great powers—eventually forming two opposing military alliances: the Allies and the Axis...

, with increasing power of smaller organizations, it may not be required to have a ground force of the size of:

• Field army
  Field army
  A Field Army, or Area Army, usually referred to simply as an Army, is a term used by many national military forces for a military formation superior to a corps and beneath an army group....
  
  , made up of two or more corps
  Corps
  A corps is either a large formation, or an administrative grouping of troops within an armed force with a common function such as Artillery or Signals representing an arm of service...
  
   plus units under the direct control of the army commander
• Army group
  Army group
  An army group is a military organization consisting of several field armies, which is self-sufficient for indefinite periods. It is usually responsible for a particular geographic area...
  
  , composed of two or more field armies
  Field army
  A Field Army, or Area Army, usually referred to simply as an Army, is a term used by many national military forces for a military formation superior to a corps and beneath an army group....
  
   plus units subordinated to the army group commander
• Theater
  Theater (warfare)
  In warfare, a theater, is defined as an area or place within which important military events occur or are progressing. The entirety of the air, land, and sea area that is or that may potentially become involved in war operations....
  
  , composed of all land, sea, and air forces in a geographic area

Even in World War II, while the Western Allies used these terms, they were not universal. A Soviet army was roughly equivalent to a US or Commonwealth corps, with a front roughly equivalent to an army group. Japanese armies were also equivalent to US or Commonwealth corps, an area army to a western field army, and a general army to a theater.

Current US concepts of EAC

The US Army divides types of troops into Combat Arms (infantry, armor, artillery, aviation, special forces), Combat Support (intelligence, communications, engineer, military police) and Combat Service Support (supply, maintenance, medical, transportation, chaplain, finance and administration).

Combat Support

Military police, in the current environment, often are in a combat role. Doctrine is catching up with reality, as in Panama, and most recently in Iraq, with the Raven 42 patrol led by Staff Sergeant Timothy Nein and Sergeant Leigh Ann Hester

Leigh Ann Hester

Leigh Ann Hester of the 617th Military Police Company, a Kentucky Army National Guard unit out of Richmond, Kentucky, received the Silver Star for her actions on March 20, 2005 during an enemy ambush on a supply convoy near the town of Salman Pak, Iraq.Hester was the first female soldier to...

, where a woman, for the first time, received the Silver Star

Silver Star

The Silver Star is the third-highest combat military decoration that can be awarded to a member of any branch of the United States armed forces for valor in the face of the enemy....

 medal, and qualified for the Combat Action Badge

Combat Action Badge

The Combat Action Badge is a military badge worn in the U.S. Army. The emblem features both an M9 bayonet and M67 grenade. The Combat Action Badge may be awarded to any soldier after the date of September 18, 2001 performing duties in an area where hostile fire pay or imminent danger pay is...

. Raven 42, a unit of the 617th Military Police Company of the Kentucky National Guard, was on a convoy escort mission where combat was reasonable to expect, as opposed to the rear area MP units guarding prisons and POWs.

Combat Service Support

The author, a Quartermaster Corps major at the time of its writing, analyzes the roles of combat service support at EAC. It must be remembered that US Army doctrine was in flux as this monograph was being developed, analyzing the lessons learned from Operation Desert Storm, but the Army not yet gone to the major restructuring into Units of Action/Brigade Combat Teams and Units of Employment.
[from the abstract]

Army concepts for supporting operations involving multiple corps were called into question by actions taken during Operations Desert Shield and Desert Storm. Non-doctrinal organizations provided support to U.S. forces deployed on these operations, despite the fact that doctrinally correct organizations existed. The monograph examines existing doctrine for support of multi-corps operations, and also doctrine for Army theater command and control architecture. It then summarizes support operations during the Gulf
War Emerging logistics doctrine is then compared with both previous doctrine and with lessons learned in the Gulf. The monograph concludes that existing doctrine for support at echelons above corps requires revision, that-emerging doctrine is on the right track, and that a key requirement will be for the Army to identify a way to test new doctrine in a realistic manner.

Is there an EAC between US corps and unified command?

Still in use in US forces are unified combatant command

Unified Combatant Command

A Unified Combatant Command is a United States Department of Defense command that is composed of forces from at least two Military Departments and has a broad and continuing mission. These commands are established to provide effective command and control of U.S. military forces, regardless of...

s, roughly equivalent to a theater. Multinational Force Iraq (MNF-I) is a level of command comparable to a reinforced field army.