The incident happens in chapter 4 of Buried Onions. Eddie's been hanging out with his buddy José, who's on home leave from the Marines. One morning, they're having breakfast, and after they come out of the restaurant, Eddie sees Mrs. Stiles's stolen truck—the one that he was supposed to be looking after—parked across the street. He quickly heads back inside the restaurant to phone Mr. Stiles and tell him what's happened. When he comes back out, he's shocked to see that José's been stabbed by three guys, who quickly run off when Eddie approaches them.
Eddie notices that José's blood on the sidewalk is in the shape of the United States. This could be said to symbolize the service that José gives to his country by serving as a Marine. It also highlights the fact that José, unlike most young men of the barrio, has been able to escape. His whole outlook on life is thus much broader as a result. His horizons are shaped no longer by the neighborhood in which he grew up but by his role in the military. As such, he has an attachment to his country that most of the young men he grew up with, alienated as they are by poverty, racism, and gang violence, have never really had.