Vilnius Region
The Lithuanian–Polish relations during the interbellum period were strained since both sides had laid claim to the Vilnius Region. During World War II, these tensions were exacerbated by different allegiances: Lithuanian administration and paramilitary units were leaning towards Nazi Germany while Polish resistance waged an active partisan war against the Nazis. Eventually, these tensions grew into a low-level civil war that culminated in a series of civilian massacres.