The following are events related to the Global War On Terrorism (GWOT) that took place on this day.

2003

• Start of Operation Rifles Fury in Iraq

2004

• The 11th MEU assumed operational control of Karbala province from the Polish-led Multi-National Division Central South. The Marines conducted security patrols, civil-military operations, and helped train, equip, and build leadership in the 401st Iraqi National Guard Battalion in the area southwest of Baghdad.
• Start of Operation Backbreaker in Iraq
• Operation Lion Cub in Iraq

2005

• Marines with I MEF began deploying to Iraq to relieve the Marines of II MEF in the fourth large-scale deployment the Marine Corps had undertaken in the past two years in support of Operation Iraqi Freedom. More than 25,000 Marines and sailors with I MEF were expected to be in western Iraq by January 2006.

2006

• Eight Marines were charged in the killings of 24 Iraqi civilians in the town of Haditha in November 2005. Four of the Marines, all enlisted, were charged with unpremeditated murder while four officers, who were not present during the actual incident, were accused of dereliction of duty for failures in investigating and reporting the deaths.
• End of Operation Moonlight (Alkamra Almaner) in Iraq

Casualties on December 21:

• CHECO, STEVEN – December 21, 2002
• AYRO, LIONEL – December 21, 2004
• BALDWIN, JOEL EGAN – December 21, 2004
• CASTRO, JONATHAN – December 21, 2004
• DOSTIE, THOMAS JOHN – December 21, 2004
• HEWITT, CORY MICHAEL – December 21, 2004
• JACOBSEN, WILLIAM WADDOUPS JR – December 21, 2004
• JOHNSON, ROBERT SCOTT – December 21, 2004
• KARPOWICH, PAUL DANIEL – December 21, 2004
• MASON, NICHOLAS CONAN – December 21, 2004
• MELO, JULIAN S – December 21, 2004
• ODELL, ROBERT DANIEL – December 21, 2004
• PETSCHE, NEIL DAVID – December 21, 2004
• POULIN, LYNN ROBERT SR – December 21, 2004
• RUHREN, DAVID ALAN – December 21, 2004
• VANKOMEN, DARREN DALE – December 21, 2004
• BURGESS, RYAN JOHN – December 21, 2006
• MAYHAN, RYAN LEE – December 21, 2006
• NOLEN, KYLE ANTHONY – December 21, 2006
• TAMAYO, FERNANDO SANCHEZ – December 21, 2006
• HOWELL, GEORGE JIMMIE – December 21, 2007
• REILLY, THOMAS JR – December 21, 2008
• CROUSE, WILLIAM HARRY IV – December 21, 2010
• BRAGG, MIKAYLA ANNE – December 21, 2011
• BONACASA, LOUIS MICHAEL – December 21, 2015
• CINCO, MICHAEL ANTHONY – December 21, 2015
• LEMM, JOSEPH GERARD – December 21, 2015
• MCBRIDE, CHESTER JAMICHEAL – December 21, 2015
• TAUB, PETER WAGNER – December 21, 2015
• VORDERBRUGGEN, ADRIANNA MARIA – December 21, 2015