In the wake of the Ft. Hood shootings by Major Nidal Malik Hasan, an uncomfortable and unjust spotlight has been trained on Muslim-American soldiers serving in the armed forces. To be blunt, their loyalties are being questioned, with some conservatives arguing that Muslims should be barred outright from serving in the military, invoking the analogy to World War II that we did not commission "Japanese nationalists or Nazis." The implicit premise is of course that an ordinary Muslim soldier is akin to a Nazi in terms of ideological loyalty. In truth, however, ordinary Germans and Japanese Americans did indeed serve in World War II with honor and distinction -- just as Muslim-Americans serve today.
However, as Islam is a creed rather than an ethnic background, one can reasonably ask whether there is any conflict with the demands of identity between faith and service. The concept of "Ummah," or community of believers, is one nearly every Muslim believes in a symbolic sense, though I question its pragmatic meaning. Much like the term, "the West", the Ummah is amorphous and has no formal authority. Muslims in Xianjing province and in Hawaii (and all Muslims in between) have default membership in the Ummah by virtue of shared faith, but to what extent do Muslims, so far separated, actually communicate or interact in any meaningful sense? How can such a vast entity have any cohesion? The sole occassion where the concept of Ummah has any genuine meaning is during the Hajj, where Muslims from every corner of the globe unite in pursuit of piety and prayer. But this too, is fleeting. Muslims who sat side-by-side in front of the Kaaba during Hajj share a bond of experience, but after Hajj ends they go back to being cardiologists in Los Angeles or street sweepers in Bangladesh and that bond is, for all intents and purposes, severed.
The Koran, however, is clear -- Muslims should not kill other Muslims. There are three verses in particular, [4.92-93] and [17.33],
[4:92] Never should a believer kill a believer; but (If it so happens) by mistake, (Compensation is due): If one (so) kills a believer, it is ordained that he should free a believing slave, and pay compensation to the deceased's family, unless they remit it freely. If the deceased belonged to a people at war with you, and he was a believer, the freeing of a believing slave (Is enough). If he belonged to a people with whom ye have treaty of Mutual alliance, compensation should be paid to his family, and a believing slave be freed. For those who find this beyond their means, (is prescribed) a fast for two months running: by way of repentance to Allah: for Allah hath all knowledge and all wisdom.
[4:93] If a man kills a believer intentionally, his recompense is Hell, to abide therein (For ever): And the wrath and the curse of Allah are upon him, and a dreadful penalty is prepared for him.
[17:33] Nor take life - which Allah has made sacred - except for just cause. And if anyone is slain wrongfully, we have given his heir authority (to demand qisas or to forgive): but let him not exceed bounds in the matter of taking life; for he is helped (by the Law).
Verse [4.93] is often quoted without [4.92] as essential context, and taking the two together there does seem to be an exception for a "people at war with you" as long as the soldier does pay penance. It is quite possible for a soldier to invoke [4.92] as permiting them to kill Muslims in the line of duty of those Muslims were at war with the U.S. One could argue that [4.93] explicitly threatens you with hell if you kill a Muslim, but that reading is only supported if you ignore the immediately preceding verse. My own take -- and I am not a scholar -- is that [4.93] applies to those cases not covered by [4.92]. And let's also note that "a believer" can also mean Jews and Christians, as explicitly stated in the Constitution of Medina by the Prophet SAW himself.