At the start of 1939, the British Army was, as it traditionally always had been, a small volunteer professional army. At the beginning of the Second World War on 1 September 1939, the British Army was small in comparison with those of its enemies, as it had been at the beginning of the First World War in 1914. It also quickly became evident that the initial structure and manpower of the British Army was woefully unprepared and ill-equipped for a war with multiple enemies on multiple fronts. During the early war years, mainly from 1940 to 1942, the British Army suffered defeat in almost every theatre of war in which it was deployed. But, from late 1942 onwards, starting with the Second Battle of El Alamein, the British Army's fortunes changed and it rarely suffered another defeat.

The Special Air Service (SAS) is a special forces unit of the British Army. It was founded as a regiment in 1941 by David Stirling, and in 1950 it was reconstituted as a corps. The unit specialises in a number of roles including counter-terrorism, hostage rescue, direct action and special reconnaissance. Much of the information about the SAS is highly classified, and the unit is not commented on by either the British government or the Ministry of Defence due to the secrecy and sensitivity of its operations.