I would guess it is not something difficult to do in this scenario. We are talking about a war where young men get drafted and sent to the carnage, most of them

I would guess it is not something difficult to do in this scenario. We are talking about a war where young men get drafted and sent to the carnage, most of them don't go there voluntarily out of some ideology. So it's easy to see some years later that you were just being used and form a bond with someone that was exactly in the same situation as you were but using different colors. All in all, those two men had more in common with each other than with the men from their own country that were waging war from whiting their palaces and sending them to their deaths.