What is an expeditionary force?
Is it a group of three to four ships enforcing a mission against fishermen?
Is it three or four combat aircraft loitering around?
Is it forces where a war on land on a neighbour that is waged? For instance was the Sino Vietnam War fought by expeditionary forces?
Expedition means an organised journey or voyage for a specific purpose, esp for exploration or for a scientific or military purpose.
So, what does expeditionary force mean?
It is obvious that such a force is to effect its mission on foreign soil that is not contiguous to the nation executing such a force.
The first core issue is expeditionary readiness. A military ethos that supports an instant response at short notice, the force being ready to respond quickly and which can transit from a peacetime to an operational readiness at a moment's notice without reconstituting its elements or augmenting itself with reserves from elsewhere. Such a force should also be in a position and organisation for mission accomplishment.
It also demands of such a force to operate successfully in an ambiguous and uncertain environment, having inherent internal flexibility to respond to the changing combat and operating scenarios.
It is obvious that such a force must have the entire elements essential to accomplish the mission. Hence, it must be organised on a combat arms operation format to include all the land, naval and air elements so as to synergies the combat power and combat flexibility. It goes without saying, it has to have one Supreme Commander and a multifaceted Staff that can orchestrate the combat as a unified whole.
Composition
The four core elements of the Expeditionary Task Force (ETF) are :
 The Command Element (CE), a headquarters unit that directs the other elements.
 The Ground Combat Element (GCE), usually comprisinginfantry, supported by armour (tanks), and artillery, but may also include special units such as scouts or Recon Battalions,snipers, and forward air controllers.
 The Aviation Combat Element (ACE), which contributes theair power to the ETF. The ACE includes all aircraft (both fixed wing and helicopters), their pilots and maintenancepersonnel, and those units necessary for aviation command and control.
 The Logistics Combat Element (LCE), contains all of the support units for the ETF: communications, combat engineers, motor transport, medical, supply units, and certain specialized groups such as air delivery and landing support teams.