Sentence:  It is clear that the firing of very heavy guns, or the enemy's fire in return, would very seriously interfere with an abbatis, or anything of that kind, and it will only be something of the lightest character, or something that is placed at a considerable distance from the <WSD>friendly</WSD> fire, the fire of the gun itself, that would remain. 
Meaning of the word marked with <WSD> tags:  Of or pertaining to friendlies ("friendly" noun sense 2, below). Also applied to other bipolar confrontations, such as team sports