In the story The Interlopers, the main theme would be the enmity between Ulrich and Georg, which causes the two men to feud over the small stretch of forest land and become bitter enemies in the process. The beginning of the story is spent explaining how this dispute began, with each man's grandfather fighting over it, until the wealthy landowner Ulrich claims that the land is legally his despite the claims of Georg to the contrary. The latter continues to hunt on the land and this makes Ulrich mad enough to devise a plan to destroy him. So the two men set out on this fateful night to confront each other and end their bitter feud once and for all. Both men, due to their enmity, see the other as the interloper.
There is also a secondary theme that the author introduces which causes the surprise at the end of the story and also introduces the element of irony. Although the men claim legal and/or hunting rights to the land, the idea or theme of nature vs. man comes into play when both men, having been felled and made captive by a giant tree, discover that they are not alone in the forest nor are they in the position to defend their claims or themselves from becoming prey to the natural owners of this piece of wilderness. Despite the fact that Ulrich and Georg had finally decided to disavow their enmity and become friends after all, in the end they will realize that they have made a fatal mistake in thinking that either of them could "own" this stretch of forest land. The wolves, the true and natural owners of the wilderness, have arrived.