Thanks for the question and for the opportunity to respond. Biblically, the only justification for warfare we find is the instruction given to the Jews upon their entrance into the "Promised Land." In addition, you have the analogies given by the Apostle Paul concerning warriors, armor, and the discipline of a soldier. These all presuppose appropriateness in the endeavor.
As a general rule, killing is divided into two categories in Scripture. One is murder, when a person aggressively seeks to kill another at no provocation. The other is not murder—the defence of self or someone under your protection. As a member of the armed forces, your activity in killing is (at least in a democracy) limited to the defence of home and kin. Hence, it is not viewed as a form of murder, but is categorized under self-defence. If the United States were to become involved in some imperialism or the attempt to expand itself at the expense of an innocent country, it would be another matter. In any event, your duty as a citizen is to fulfill the command of the civil authority - cp. Romans 13:1-4. You are in essence a "sword bearer" in the hands of divinely established authority.
Hopefully this is of assistance in clarifying the issues for you.