In terms of a Masters Degree, a second Language isnt critical (from my experience). That is unless you absolutely need it for your research field. But if you want to go on and get your PhD (and you really should), you will absolutely need a second language.

Which language really depends on your subject and your department. Im a historian, so I cant specifically talk about Anthro, but generally youll have to learn the languages which relate to what youre studying. If youre interested in PsyOps in the Pacific Theater of World War Two, you are probably going to have to learn Japanese. As you narrow down what you study, youll narrow down which languages you really want to study. As another example, I study Cold War American tank development. So I could pick from a whole bunch of language to study like German, Russian, French, Dutch, or Hebrew. However, my interest in how that development affects NATO instantly cuts down my list of useful languages. From there, it becomes a list of the most useful Languages being German than French than Dutch. Since I have a background and interest in German, it makes that choice simple.

Because youve already had some experience with a second language, it would be really easy to use that to fulfill a second language. But if you dont want to study their history, its not going to help you much. The big question becomes, what do you want to study? What makes your heart go pitter patter?

The other question I would have for you is: what does anthro do for you that history cant? I cant think of many places where anthro would help you in military history over traditional history. Especially with your focus, youd fall into whats called "new" military history and which is extremely trendy right now. It would be a lot easier for you to get published in a journal than say, me, who does something a bit more traditional. It could also help with getting into a PhD later on.

I think that's a fascinating subject, and I've seen it studied from a number of angles. As just a few quick examples:

• Scott Brantner, "Imaginibus, non mente falluntur: Post-traumatic Stress in Roman Medicine and Literature, 100 BCE – 100 CE" (unpublished paper). Uses modern psychological studies of PTSD to guide a study of classical Latin texts.

• Neil Price, The Viking Way. A beautiful example of "cognitive archaeology." Price approaches the Viking Age using modern ethnographies of shamanism in Siberia as a basis for comparison. If you still have library access with your undergrad (and I'd contact your library if you're unsure), you might well be able to download this free online.

• Ben Raffield, "Bands of brothers: a re-appraisal of the Viking Great Army and its implications for the Scandinavian colonization of England," Early Medieval Europe (forthcoming). Raffield previously studied under Price. This is a very exciting example of a more focused study on the viking armies that invaded England.

• James Broomall, interview "Behind the Lines" (2014). I couldn't find anything by Broomall to read in a rapid search of the interwebs, but he's a young scholar researching how the experience of the Civil War reshaped what masculinity meant in the South.

So that's a quick sample of what can be studied in classics, anthropology (US anthro departments include archaeology), and history—if you can find the right advisor. From the sounds of it, your biggest choice will be deciding just where/when you want to study, which will guide your choice of department and languages. Anthropology departments will be especially interested in whether or not the study you propose is feasible, especially since your research will effectively be limited to what you can accomplish during one summer. For example, it might be really important for us to understand the psychological impacts of war in Syria right now, but there's no feasible way a university can send its students out to do that. This is often less of a problem with historical and archaeological approaches, especially at the MA level. For a PhD application, it would be important to show that you can safely get to your archives or archaeological sites/collections.

Practically speaking, what would I suggest? From the sounds of it, Spanish, French, or German would all be good options. Spanish could prepare you for doing oral histories or ethnographies of recent conflicts in Latin/South America, or it could prepare you for joining the vibrant field of Meso-American Archaeology. French will open up large parts of Africa and Southeast Asia for you; it could prepare you to study conflicts within France itself (e.g. the French Revolution, or more recent domestic terror); and it's a great basis for transitioning into Latin studies. German is a bear for many Americans, but I have personally found it tremendously useful as a second language. It drilled me in grammar that's helped me acquire other languages more quickly, and it's been a great bridge into other Germanic languages, both old and new (Old English, Old Norse, Danish, Swedish, and Norwegian). It's also a traditional academic language, so lots of important work is published in German and unfortunately unaccessible to far too many native English speakers. Arabic, Farsi, Turkish, Russian, and Czech might all be additional languages worth considering.

I'd also recommend reading as much history and anthro stuff as you can, and check if you have library access through your undergrad or through a local university. It would do a world of good to regularly check in on journals like the American Historical Review and the Journal of Social Anthropology. Find out what interests you, look at the people and books they're citing, and think about what kind of research and training would benefit you most in the long run. You might get some good advice from /r/AskAnthropology as well. And above all be flexible. An MA should open up whole new worlds of thinking, rather than pigeon-holing you into a research field. It's hard work, but a great adventure!