友と君と戦場で appears to be a game, where 戦場 has been given the non-standard furigana "ここ"
implying, I suppose, ここは戦場 .
As Mike said, 戦場 is battlefield, not "war".
This is a quite common technique used to add double-meanings. It doesn't mean that ここ means "battlefield" in Japanese more generally. There are a lot of non-standard uses of ruby text, for example it's not that rare to see a made up term in fantasy fiction in kanji, with furigana in katakana, e.g. stuff like 甘毒 furiganaed with スィート・ポイズン . The author wants to use the "cool sounding English" term, but also knows the readers might not get the meaning, hence combining the two.
Another interesting one is where terms in katakana are given ruby kanji "explanations". For example, a crime drama set in the US where Interpol is written インターポール in dialogue but 国際刑事警察機構 is put in ruby text the first time it's introduced. In an Agatha Christie novel in translation I have, more obscure English placenames are transcribed in katakana but then given a little explanation in ruby text. "A town north of London", or something like that. (I will try to dig up a proper quote from that one later).
Basically, ruby text/furigana can be any of:
1) Showing the proper reading (e.g. kanji are difficult for the intended audience, word uses non-standard readings of the kanji)
2) Showing an alternate/made-up reading (often in katakana)
3) Indicating a double meaning (好きな人 furiganed きみ or something) - can overlap with 2).
4) Giving clarifying information for difficult words (particularly foreign words) - like a sort of in-line footnote.
5) Probably some stuff I've missed
