The 3rd Division, also known as The Iron Division, is a regular army division of the British Army. It was created in 1809 by Arthur Wellesley, 1st Duke of Wellington, as part of the Anglo-Portuguese Army, for service in the Peninsular War, and was known as the Fighting 3rd under Sir Thomas Picton during the Napoleonic Wars. The division fought at the Battle of Waterloo, as well as during the Crimean War and the Second Boer War. As a result of bitter fighting in 1916, during the First World War, the division became referred to as the 3rd (Iron) Division, or the Iron Division or Ironsides. During the Second World War, the division fought in the Battle of France including a rearguard action during the Dunkirk Evacuation, and played a prominent role in the D-Day landings of 6 June 1944. The division was to have been part of a proposed Commonwealth Corps, formed for a planned invasion of Japan in 1945–46, and later served in the British Mandate of Palestine. During the Second World War, the insignia became the "pattern of three" — a black triangle trisected by an inverted red triangle.

The Anglo-Portuguese Army was the combined British and Portuguese army that participated in the Peninsular War, under the command of Arthur Wellesley. The Army is also referred to as the British-Portuguese Army and, in Portuguese, as the Exército Anglo-Luso or the Exército Anglo-Português.