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I have been in the army for about few months and it's a non Muslim country, for about the duration I spent here I have been eating only bread, vegetarian meals and sometimes if there were fish or tuna I'd eat them (rarely there were fish meals like once a week or twice). Today as I was normally eating my porridge and bread, a Muslim brother approached me and asked why I didn't eat the turkey slice and I told him its not halal, he then proceeded to tell me it's halal and we are muslims in non-muslim country so there is no choice.
My question here is if it is permissible for me to eat cow, chicken, deer and so on, animals that are only halal when slaughtered in Islamic way When I am in a situation like this?. Please give simple answers and quotes from hadith or Quran.