The I Marine Expeditionary Force ("I" pronounced "One") is a Marine Air Ground Task Force (MAGTF) of the United States Marine Corps primarily composed of the 1st Marine Division, 3rd Marine Aircraft Wing, and 1st Marine Logistics Group. It is based at Marine Corps Base Camp Pendleton.

This article includes a list of general references, but it lacks sufficient corresponding inline citations. (March 2018)

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I Marine Expeditionary Force
I MEF insignia
Active	8 November 1969 – present
Country	United States of America
Branch	United States Marine Corps
Type	Marine Air-Ground Task Force
Role	Expeditionary combat force
Part of	U.S. Marine Forces Pacific
Garrison/HQ	MCB Camp Pendleton, California, U.S.
Engagements	Operation Desert Storm Global War on Terrorism Operation Enduring Freedom Iraq War Invasion of Iraq First Battle of Fallujah Second Battle of Fallujah
Commanders
Current commander	LtGen Michael S. Cederholm
Notable commanders	Gen Joseph F. Dunford, Jr., Gen Anthony Zinni, Gen Michael Hagee, Gen James T. Conway, Gen James Mattis, Gen John F. Sattler

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I Marine Expeditionary Force is the largest of the three MEFs in the Fleet Marine Force and is often referred to as the "Warfighting MEF" for its consistent involvement and contributions in major armed conflicts. It is presently commanded by Lieutenant General Michael Cederholm. The deputy commander is Brigadier General Robert C. Fulford.

Pronunciation of the Roman numeral designator: As a Roman numeral the capital letter "I", representing one, is properly pronounced as "One". However, there are some who erroneously pronounce the number as either "First", or either intentionally, or unknowingly, pronounce it as "Eye", as in the letter "I".

The convention of using Roman numerals to designate a MEF, which is itself the Marine Corps equivalent organization to an Army corps, stems from the U.S. Army practice that began in the American Civil War, and continues today, of numbering corps (two or more divisions with supporting troops, and sometimes including separate brigades, regiments, groups, or battalions, all under a unified corps headquarters, usually commanded by a lieutenant general) with Roman numerals. Corps, themselves being the first-level sub-unit of a "field army", or a numbered, or named, army (e.g., First U.S. Army, or the Army of the Potomac).

During the First World War, the 4th Marine Brigade, as part of the U.S. Army 2nd Infantry Division, came under the U.S. Army I Corps, American Expeditionary Forces. With the expansion of the Marine Corps to six divisions and five air wings during the Second World War, the Marine Corps created two "Amphibious Corps", I Marine Amphibious Corps (later re-designated as III Amphibious Corps) and V Amphibious Corps, continuing the custom begun by the Army. Modern Marine Expeditionary Forces, or MEFs (for a time known as Marine Amphibious [italics added] Forces, or MABs) continue the U.S. Marine Corps legacy as corps-equivalent organizations designated by Roman numerals.

When directed, I MEF deploys and is employed as a Marine Air Ground Task Force (MAGTF) in support of Combatant Commander (COCOM) requirements for contingency response or Major Theater War; with appropriate augmentation, serves as the core element of a Joint Task Force (JTF); prepares and deploys combat-ready MAGTF's to support COCOM presence and crisis response; and supports service and COCOM initiatives as required.

• Activated on 8 November 1969 at Okinawa, Japan as the I Marine Expeditionary Force
• Redesignated on 18 August 1970 as the I Marine Amphibious Force (I MAF)
• Relocated in April 1971 to Camp Pendleton, California
• Redesignated on 5 February 1988 as the "I Marine Expeditionary Force"

• Operation Desert Shield – Southwest Asia – August 1990 – April 1991
• Operation Desert Storm – Southwest Asia – August 1990 – April 1991
• Operation Restore Hope- December 1992 – May 1993
• Operation Southern Watch – Iraq – January 1998 – February 1998
• Operation Desert Thunder – Iraq – February 1998 – June 1998
• Operation Enduring Freedom – Kuwait, Afghanistan – November 2002
• Operation Iraqi Freedom – Iraq – March 2003 – 2010