The British Armed Forces are charged with protecting the United Kingdom and its overseas territories, promoting Britain's wider security interests, and supporting international peacekeeping efforts. The 42,000-member Royal Navy is in charge of the United Kingdom's independent strategic nuclear arm, which consists of four Trident missile submarines. The Royal Marines provide commando units for amphibious assault and for specialist reinforcement forces in and beyond the NATO area. The British Army--with a reported strength of 110,000 in 2001, including 7,600 women--and the Royal Air Force--with a strength of 54,000--along with the Royal Navy and Royal Marines, are active and regular participants in NATO and other coalition operations.

The United Kingdom stood shoulder to shoulder with the United States following the September 11, 2001 terrorist attacks in the U.S., and its military forces participated in the war in Afghanistan. The U.K. was the United States' main coalition partner in Operation Iraqi Freedom and continues to have more than 8,000 troops deployed in Iraq to help stabilize and rebuild the country. Under UN Security Council Resolution 1483, the U.K. also shared with the United States responsibility for civil administration in Iraq and was an active participant in the Coalition Provisional Authority before the handover of Iraqi sovereignty on June 28, 2004. Britain's participation in the Iraq war and its aftermath remains a domestically controversial issue.