Answer:
The details that the author uses to make us feel sorry for the sentry and feel angry towards the aliens are that the aliens took over without negotiation and that he was in miserable conditions trying to survive the war. We know this from lines 13-14 where it states, "They'd shot without even trying to negotiate, or to make peace." and on line 16 where it says, "He was wet and muddy and hungry and cold.".