A Forward Operating Site (FOS) or Forward Operating Location (FOL) is a U.S. military term for facilities, defined as "a scalable, 'warm' facility that can support sustained operations, but with only a small permanent presence of support or contractor personnel. A FOS will host occasional rotational forces and many contain pre-positioned equipment." These sites were established as the Pentagon

The Pentagon

The Pentagon is the headquarters of the United States Department of Defense, located in Arlington County, Virginia. As a symbol of the U.S. military, "the Pentagon" is often used metonymically to refer to the Department of Defense rather than the building itself.Designed by the American architect...

 began to address regional threats primarily in Africa

Africa

Africa is the world's second largest and second most populous continent, after Asia. At about 30.2 million km² including adjacent islands, it covers 6% of the Earth's total surface area and 20.4% of the total land area...

 and Latin America

Latin America

Latin America is a region of the Americas where Romance languages  – particularly Spanish and Portuguese, and variably French – are primarily spoken. Latin America has an area of approximately 21,069,500 km² , almost 3.9% of the Earth's surface or 14.1% of its land surface area...

 following its 2004 global posture review.

An FOS is differentiated from a Cooperative Security Location

Cooperative Security Location

A Cooperative Security Location is a U.S. military term for facilities used for regional training in counterterrorism and interdiction of drug trafficking, and also to provide contingency access to the continent. "A CSL is a host-nation facility with little or no permanent U.S...

 (CSL) with no permanent force or contractor personnel, or a Main Operating Base

Main Operating Base

Main Operating Base is a term used by the United States military defined as "an overseas, permanently manned, well protected base, used to support permanently deployed forces, and with robust sea and/or air access." This term was used to differentiate major strategic overseas military facilities...

 (MOB), with a large force and a well-defended site.

Europe

• RAF Fairford
  RAF Fairford
  RAF Fairford is a Royal Air Force station in Gloucestershire, England. It is a standby airfield, not in everyday use. Its most prominent use in recent years has been as an airfield for United States Air Force B-52s during the 2003 Iraq War, Operation Allied Force in 1999, and the first Gulf War in...
  
  , United Kingdom
  United Kingdom
  The United Kingdom of Great Britain and Northern IrelandIn the United Kingdom and Dependencies, other languages have been officially recognised as legitimate autochthonous languages under the European Charter for Regional or Minority Languages...
  
  
• Bulgaria
  Bulgaria
  Bulgaria , officially the Republic of Bulgaria , is a parliamentary democracy within a unitary constitutional republic in Southeast Europe. The country borders Romania to the north, Serbia and Macedonia to the west, Greece and Turkey to the south, as well as the Black Sea to the east...
  
   (see List of joint US-Bulgarian military bases)
• Romania
  Romania
  Romania is a country located at the crossroads of Central and Southeastern Europe, on the Lower Danube, within and outside the Carpathian arch, bordering on the Black Sea...