What is the subject matter and significance of "Mending Wall"?
"Mending Wall" is about the borders, literal and metaphorical, that separate nations and individuals. The speaker in the poem questions the idea that "good fences make good neighbors." The speaker's neighbor, who repeats this refrain, is described as moving "in darkness," meaning that he moves in ignorance, refusing to question the truth of the saying that "good fences make good neighbors." The speaker's neighbor is also described dismissively as "like an old-stone savage," implying that his opinions about walls and fences are, like him, primitive, or "savage."
In other parts of the poem, the speaker suggests that walls and man-made borders are not natural. Indeed, the wall between himself and his neighbor is destroyed by "the frozen-ground swell under it," and then by "hunters," and also, continually, by a mysterious "Something . . . that doesn't love a wall." The wall is destroyed repeatedly and inevitably, so that the speaker and the neighbor must continually, out of habit it seems, meet to repair it. It seems also that they only meet, ironically, when they come together to repair the wall meant to divide them. And each time they meet, the speaker's neighbor unthinkingly insists that "good fences make good neighbors." The speaker wants the neighbor to ask himself, "Why do they make good neighbors?" The question is partly rhetorical, and the answer that the speaker means to imply is that, simply, they don't.
To understand the significance of this poem, it is important to consider it in the context of the time in which it was written. "Mending Wall" was the first poem in Frost's 1915 collection of poems entitled, North of Boston. This places it, contextually, at the beginning of World War One, which began in 1914. This was a war fought over national borders, and a war which fundamentally changed the borders of Europe.
This poem might also have significance in a more modern context too. One of Donald Trump's key election pledges to the American people in 2016 was that he would build a wall between America and Mexico. If Robert Frost was still alive, he might want more people to ask of Trump's proposed wall, as the speaker asks of his neighbor in the poem, "Why (do walls) make good neighbors?"