Serving in the US Military
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Answer
Shalom!
Thank you for your question.
While it is ideal to avoid army service when it will cause one to violate Jewish law, such as the observance of Shabbat, Kashrut, and most everything else, a nything done in the course of this service, even the need to kill sometimes, are done in the context of saving lives of one’s countrymen and are permitted.
Even a Kohen who kills in the course of self-defense or otherwise saving lives does not lose his privileges as a Kohen even though a Kohen who murdered is largely disqualified from serving as a Kohen in the future.
Military (and self-defense) are exceptions to the rules of murder, as it says in the Talmud (Berachot 62b), “If one comes to kill you, rise up and kill him first.”
As such, your fiancée is not violating the transgression of "Lo- Tirtzach". He should continue to be careful to not unnecessarily violate Jewish law.