How can we relate WWI to "The Interlopers"?
This is a brilliant question. Let us remember that this short story was published in 1919 just after the end of World War One, and obviously we can expect Saki's literature to have been impacted greatly by this seismic historical event. What I find interesting is the way that we can view this short story as an allegory about the futility of war and discord between humans. Let us remember that the beginning of the story paints an incredibly depressing picture about the capacity of humans to cling on to feuds even when the cause is slight:
The forest lands of Gradwitz were of wide extent and well stocked with game; the narrow strip of precipitous woodland that lay on its outskirt was not remarkable for the game it harboured or the shooting it afforded, but it was the most jealously guarded of all its ownder's territorial possessions.
Even though the land is almost worthless, still Gregor and Ulrich are prepared to kill each other to own it. We can perhaps see their feuding as an allegory of the two different sides of the war. Both fight viciously and tenaciously for little reason. The story gives us a grim and ironic warning by showing the way in which Gregor and Ulrich become friends, but only when it is too late, for nature and the world seems to have tired of this feud that is being fought over land, and has acted itself to end the feud in its own way. The message of this story seems to be, that if we devote ourselves to hating another human, we ultimately cause our own extinction and demise.