Of all the means of becoming a citizen one of the most interesting is the acquisition of citizenship through military service. It used to be that such service was the duty of a citizen himself but today most Western democracies no longer include this as a duty of citizenship and their militaries are purely voluntary organizations. Conscription in Australia ended in 1972, in 1973 in the U.S., in 1994 in Belgium and so on.
It lasted a bit longer in France which is ironic because she is often considered the birthplace of modern mass conscription which was implemented during the French revolution. The draft in the Hexagone ended in 2001 and my spouse, his brother and his brother-in-law all served before it was finally abolished by President Jacques Chirac. I remember my father-in-law, a retired French military officer, having a number of misgivings about this. He led, at the end of his career, the Fire Department of Paris which is part of the French Army and he was deeply concerned about how the end of the draft would affect that institution.
Military service and citizenship have always had a close relationship. Citizenship can define who can or cannot serve and, in some cases, citizenship laws are changed in order to make more individuals eligible. For example, it is generally accepted that in a democratic nation-state you cannot oblige non-citizens to serve in the military even if they are long-term residents. So, a country that has a large population of foreign nationals (some of whom may have lived in the country for generations) cannot count on them to defend the nation in which they reside. This has been an important issue in countries that based citizenship on jus sanguinis (blood) and deprived women of their citizenship if they married foreigners (their children took their foreign father's citizenship). It was for this reason that Napoleon wanted citizenship to be based on jus soli (right of soil) - he wanted to ensure that young men born in France could not escape conscription.
He lost the debate, citizenship from 1803 on was based on jus sanguinis, and what he predicted did indeed come to pass. Patrick Weil describes some surreal situations where the French Army would come into a village to levy troops and half the young men (born in France, mind you) would magically produce their Italian/Spanish papers and be exempt from service. Understandably this action infuriated the purely French families in the village who were then obliged to provide more of their young men to make up for the lack. It was for this reason that double jus soli became the law of the land - someone born of foreign parents who were themselves born in France automatically became a French citizen at birth and that was the end of that.
In the times of the European colonies the citizenship issue was a bit different since there was a real question of whether or not to allow natives of the colonized countries to serve in the military of the colonizing empire. Allowing them to do so created a kind of debt on the part of the mother country which could have (and sometimes did) lead to demands for greater rights within the empire and possibly full citizenship for those who served. In World War II over a million Indians and 120,000 West Africans fought for the British. French forces that fought in Europe during that period included 12 Armees d'Afrique with many units composed of North Africans. According to Thomas Janoski in his book, The Ironies of Citizenship: "in the late 1950's the composition of the [French] armed forces was only 31% European French, with the rest coming from or in the colonies - 30% Indo-Chinese, 17% North African, 11% Foreign Legion (mainly other Europeans) and 10% Central Africans and Madagascans."
In more recent times with more and more nation-states moving toward voluntary military service there is still that link with citizenship. The United States of America allows for expedited naturalization though military service and it is estimated that there are 30,000 non-US citizens serving in the U.S. military, many of them in Iraq and Afghanistan. France still has its famous French Foreign Legion and a foreign soldier has the right to French citizenship after 3 years of service. Interestingly enough two countries that did not accept foreigners in their armed forces seem to be reconsidering the question. Earlier this year the German Defense Ministry published a report which call for the end of conscription in Germany and a plan to recruit foreigners residing in Germany. The Russian Defense Ministry recently announced the creation of a Russian Foreign Legion which also allows for the acquisition of citizenship after 3 years of service.
In general most countries (but not always their citizens) see this method of acquiring citizenship as being entirely legitimate. An individual who serves a nation and risks his or her life on its behalf is viewed as someone who has earned the right to become a full citizen. However this does raise a question concerning native-born citizens who, in many cases, are accidental or involuntary citizens. They have done nothing concrete to earn their citizenship since it was simply conferred on them as an accident of birth. For some reason (the origin of which I dare not speculate on here), it is often a loud minority of these people who argue most strenuously against immigration and naturalization in all its forms.
To those few (but often very loud) natives who argue against naturalization by service or who wish to limit the rights of voluntary citizens, I would ask this question: do you live in a democracy or an aristocracy? If the answer is the former and not the latter, then the acquisition of citizenship through service to a nation is not only just and right, it is the highest expression of true democratic principles because it explicitly connects the rights of a citizen with service to a nation-state.
And, from my perspective, the willingness to spill the blood that is in your veins in the service of a nation-state should always trump blood merely inherited from one's parents.