In WW2, almost all commonwealth country and allied forces has taken a SMLE rifle for their standard side arm for their troops.
The Lee-Enfield series of rifles was born in 1895 as a marriage between the magazine and bolt action, designed by the J. P. Lee, and the new pattern of barrel rifling, designed at the Royal Small Arms Factory (RSAF) at Enfield. Originally known as Lee-Metford, this design was adopted by British army in 1888 and used a Metford pattern rifling with shallow groves, intended to be used with ammunition loaded with black powder. Introduction of the smokeless powders in the form of the Cordite showed that the Metford rifling was very short-living, so it was soon replaced with Enfield rifling, with 5 traditional land and grooves and left hand pitch.
Early Lee-Enfield rifles, officially known as a ".303 caliber, Rifle, Magazine, Lee-Enfield", were carried by the British army through the Boer war (South Africa) of 1899-1902, and Boers, armed with their Mausers, taught to the Brits some hard lessons. And, unlike some other Empires, Brits were quick lo learn. In 1903, they introduced a new design, which improved over the older Lee-Metfords and Lee-Enfields in some important respects. The main improvements was the introduction of the "universal" rifle idea.
The common thinking of the period was to issue the long rifle for infantry and the carbine for cavalry, artillery and other such troops. The Brits decided to replace this variety of sizes with one, "intermediate" size, that will fit all niches. This "one size fits all" rifle was called ".303 caliber, Rifle, Short, Magazine, Lee-Enfield, Mark 1", or, in short SMLE Mk.I, where "short" referred to the length of the rifle. This rifle passed some improvements during the following pre-WW1 years, finalizing in the 1907 as a SMLE Mk.III.
Development and introduction into service of this rifle was accompanied with constant complaints of some "theorists", which stated that this rifle would be no good neither for infantry, nor for cavalry, so RSAF was set do design another rifle, patterned after the German Mauser, which also should be more suitable for mass production, than the SMLE. This rifle finally appeared in 1914 as an ".303 caliber Enfield Pattern 1914 rifle", or simply a P-14. With the outbreak of the Great war British troops were still armed with the "poor" SMLE Mk.III rifles, which soon turned far from any "poor", giving some hard time to the Germans. In fact, the SMLE Mk.III was a really good rifle, quite accurate, reliable and suitable for rapid and accurate firing. British soldiers were rigorously trained for both individual and volley fire marksmanship, and were routinely capable of firing 30 aimed shots per minute, which was quite a rate of fire for any non-automatic rifle. There were times when advancing Germans were impressed that they were under the machine gun fire, when Tommie used their salvo-firing techniques.
During the war time the basic Mk.III design was slightly simplified to better suit the mass production needs, with omission of "volley" sights and magazine cutoffs, and with some production shortcuts. When the World War One was over, there were no questions of quality of basic SMLE design, but some improvements were suggested and introduced in later patterns, such as peep-hole, receiver mounted sights. These "interwar" patterns were not issued in any significant quantities until the 1941.
In 1926, Britains, quite confused with numerous 'Marks' and 'Marks with stars' of their weaponry, decided to adopt a new numbering system, so the SMLE Mark III became the "Rifle, No. 1 Mark 3". The "Rifle No.2" was a training version of the SMLE No.1 but chambered to .22LR ammunition. The "No.3" was assigned to the P-14 rifle, which was used in limited numbers. And the "Rifle No.4 Mark 1", widely known as a SMLE No.4 Mk.1, appeared in 1941. This was an improved and strengthened SMLE design, with heavier and stronger receiver, which also was faster and easier to machine, and with heavier barrel. The stock shape was shortened at the front part, giving away with the characteristic Mark III snub-nosed appearance. The barrel-mounted open rear sights were replaced with the receiver-mounted peep-hole sights, which were micrometer-adjustable.
The latter feature was substituted by the simplified flip-up rear sights for wartime production, and this version became the No.4 Mk.1* rifle. By the end of the World War 2, when British and Commonwealth troops (also armed with SMLEs) started to fight in jungles of the South-East Asia, it was soon discovered that a "short" SMLE was still not short enough for the jungle combat, so a carbine version was adopted late in the 1944 in the form of the No.5 "jungle carbine". This gun was somewhat lighter and handier than No.4, but suffered from the "wandering zero" problems, which meant that the point of impact wandered during the time. The muzzle flash and recoil were also too strong, despite the flash-hider and rubber buttpad.
The last, and by some opinions the finest "general issue" version of the SMLE was the No.4 Mk.2 rifle, which appeared in 1949. It was made by higher peacetime standards of fit and finish, than a wartime No.1 Mk.3s and No.4 Mk.1s, and served with British army until the mid-1950s, when the self-loading L1 SLR (semi-auto copy of the Belgian FN FAL) rifle in 7.62mm NATO was introduced into general service. But some SMLEs were left in military service, as a training, target and, especially, sniper rifles, known as Enfield L39 and L41, rechambered to the new standard 7.62mm NATO ammunition, and served well until the late 1980s, when there were replaced by the L96 sniper rifles. It should be noted, that SMLE rifles were produced and used not only in the UK. Australian, Canadian and Indian factories turned out more than million of the No.1 rifles with various improvements, which were used during both World wars and thereafter.
During the WW2, Britain also acquired quantities of SMLE No.4 (marked No.4 Mk.1*) made under contract at the Savage Arms company in USA. In the 1950s, Indian Isaphore arsenal turned out some SMLEs rechambered to the 7.62mm NATO (.308 win) ammunition. These are distinguishable from .303 caliber rifles by the more squared outline of the magazine. Total numbers of all 'Marks' and 'Numbers' of the SMLE made during the 60 years in various countries is not less than 5 000 000 (yes, five millions) rifles
In general the SMLE were ones of the best bolt action battle rifles, fast-firing, powerful and reliable. While being less suitable for "sporterizing" than Mausers, they are still popular among civilians as a hunting and plinking weapons, and also as a part of the history. The key deficiencies of the SMLE were probably the rimmed ammunition and non-interchangeability of bolts, but the advantages of this design were mush bigger and Lee-Enfields in all its guises served the Britain and the British Commonwealth for more than 60 years in the front line service and much longer as a specialized weapon (training and sniper).