Previously, Rin said something like "No, I definitely wouldn't be alive today if I wasn't a vampire." Combined with this chapter, we start to get a bit of his backstory.
Hildebrand likely drank blood from a dying man (Rin). By sheer chance, this happened to result in the creation of a new vampire and Rin lived. Based on the scenery and Rin's age (approximately 100 years old), I speculate that this is the scene of a battle in World War 1. That is possibly a stretch as Japan wasn't strongly involved in World War 1, and Rin does seem to be Japanese. However, Rin's body is heavily scarred with what seem to be bullet or shrapnel wounds, so that part does line up.
Edit: Actually the 100 year thing is from Rin's birth, not his death. This means the war in question would be World War 2, which makes a lot more sense as some combat did happen in Japan.