Although most people think the British Army is simply the land forces of the United Kingdom, the situation is far more complex, and is often to the advantage of the British themselves, particularly when it comes to recruiting. Although the British Empire no longer exists and places like Australia, New Zealand, and Canada are legally sovereign nations, they aren't, in a way. The Commonwealth of Nations that has replaced the legacy of British imperialism continues to provide Britain with a complex, but still strong relationship with its former dominions and for the past century, even before the Empire's demise, has continued to use this special relationship to its military advantage.
Although sovereign countries, Commonwealth Nations, especially Australia and New Zealand, continue to recognize the British Crown as their heads of state and incorporate the British flag into their national emblems. Commonwealth armies are organized, trained, and led according to largely British traditions, with a heavy responsibility for non-commissioned officers and regimental sergeant majors. But this link with the Crown goes beyond mere symbols. Both Australia and New Zealand have their own Special Air Service Regimen, modeled after their British counterparts. Also, soldiers from Commonwealth countries are permitted to transfer their services over to that of the main United Kingdom Army and vice versa. Therefore, you have Australian soldiers transferring to the British Army and British Soldiers transferring to the Australian Army. This type of service swapping has been common for decades.
The often confusing nature of the British and Commonwealth military forces means that not only do many, many countries swap soldiers on a regular basis, but that regiments are often "segregated" and even recruiting from countries who nominal links to the Crown expired decades ago. In the British Army, regiments are the building blocks of the infantry and are grouped according to their places of origin and national make up. Thus, there are regiments for Scottish, Irish, English, Nepalese, and Welsh troops. Unlike past segregation in the US Army, this type of organization is not viewed as discrimination but as a source of pride for the regiment's soldiers and calls to disband certain regiments, especially those of Scottish origins, would and have ignited intense protests.
For the Irish regiments, largely recruited from Northern Ireland, enlistments are not limited to males from the North. The Republic of Ireland cut its links with the Crown in the 1940s but, despite this, a sizeable portion of Irish Citizens serve in the Irish Guards. Basically, there is an understanding between the two governments that Irishmen are permitted to enlist in the Guards on their own initiative, but the Guards are not allowed to actively recruit in the Republic. The situation of close links between Commonwealth countries and their British allies is unlikely to fade away anytime soon, there is simply too much history and tradition associated with it and the cooperative links between the nations have proven far to beneficial in the past, particularly when major wars break out.
The British Army during major war time has never been "just British". During both World Wars, Australian and New Zealand troops were amalgamated into ANZAC (Australia New Zealand) Army Corps units which were often under the command of regular British Army generals, alongside their Irish, Indian, Canadian and South African compatriots. During the Malayan Emergency, Rhodesian SAS troops operated alongside their British allies and, since demise of the Rhodesian Republic, the designation of "C" Squadron is left unmanned out of respect for the lost Rhodesian element, sort of like retiring a jersey. This tradition provides the Commonwealth with a kind of loosely organized, transnational military force capable of banding together and defeating larger enemies should another major war break out. That kind of protection is hard to find elsewhere and the Crown is, unsurprisingly, not interested in giving it up.
Currently, soldiers from Commonwealth origin countries make up around 6 percent of the British Army's total strength, with recruits from the Republic of Ireland growing at a massive rate in recent years from around 5 percent to almost 20 percent. Some British politicians feel that the growing involvement of Commonwealth soldiers in the regular forces is diluting the national identity of the Army, but this ignores the fact that "British national identity" has, over the centuries, long been a fluid concept, particularly when the multiple nations swear allegiance to Britain's monarch. -- Rory Walkinshaw