One example of a friendship lost in Tim O'Brien's The Things They Carried can be seen in the friendship between Lee Strunk and Dave Jensen, detailed in chapter 5, titled "Enemies."
One morning while on patrol, the two friends became embroiled in a fistfight because Jensen's jackknife had gone missing, and he blamed Strunk for the loss. The fistfight became so intense that Jensen mercilessly hit Strunk's nose repeatedly, breaking it. Strunk had to be sent by helicopter to the medical unit to get his nose treated.
When he returned a couple of days later, Jensen started fearing Strunk would take revenge. Jensen's fear of his former friend's revenge got so bad that he even started having difficulty sleeping. Jensen became so crazed by his obsessive fear of revenge that he one day he started shooting his machine gun into the air, screaming Strunk's name and forcing the rest of his platoon to take cover. He then sat down, holding "his head in his arms," unable to move for three hours. Later that night, he took a pistol, whacked his own nose with it, and broke it. He then went to Strunk and asked if things were now even between the two of them. Strunk agreed they were even but couldn't stop laughing at his friend because, after all, he stole the jackknife. In other words, in Strunk's mind, a severely broken nose was justified punishment for having stolen a jackknife.
This scenario tells of a friendship that was broken all because a jackknife was stolen. But, most importantly, it shows just how much war changes relationships. War makes a person feel like everyone is an enemy. It's due to this feeling that Jensen became fearful of revenge, and we might even be able to attribute the feeling to Strunk's motive for thinking he had to steal the jackknife--he stole it because he felt he had to. Or, even if stealing the jackknife had just been a joke, we can attribute Jensen's excessive response to the feeling that everyone is an enemy. In normal situations, Jensen would probably have laughed off the instance and not have become so enraged.
Hence, the story shows us that war makes friendships very delicate because war makes one feel like everyone is an enemy.