Two lines are enough for Robert Herrick to overturn a comfortable assumption: that a city's chief danger is the stranger at the gate. "Great cities seldom rest; if there be none / T' invade from far, they'll find worse foes at home." The title, "Peace Not Permanent," states the thesis flatly, and the couplet explains the mechanism. Peace fails because a powerful community seems unable to stop producing enemies for itself, whatever the outside world does.
The couplet is built like a miniature argument: a general claim, an "if" clause, a consequence. The "if" imagines the best case, with every external threat removed, and then denies that this brings calm. The key phrase is "they'll find." Herrick doesn't say the cities will suffer or meet trouble, only that they will find it, as though they go looking. Restlessness needs an object and will settle for whatever is nearest. This is a sharper idea than the proverb that peace is fragile. The poem suggests that the energy that makes cities great, whether ambition, appetite or rivalry, doesn't switch off when the danger does. It turns inward, rather like a body attacking itself.
The word choices reward a slow look. "Great" opens the poem with a hint of irony, since size and power are the first things named and the rest of the couplet reads as their cost. "Seldom" is careful. Herrick doesn't say "never," and the hedge makes the line sound like observation, the voice of someone who has watched how such things tend to go, rather than a curse. "Rest" is well chosen too. It is what a tired body needs, so the cities become sleepless creatures. The semicolon falls right after the word, so the line pauses on "rest" and then rushes on without it.
The sentence also spills over the line break, "none / T' invade," instead of stopping politely at the end of the line. That enjambment is a small enactment of the poem's subject, since the grammar won't settle either. Both lines are iambic pentameter, ten syllables each. The apostrophe in "T' invade" squeezes "to invade" into two syllables to keep the count, and the squeeze makes the phrase brisk and almost military. In the opening line, "Great" is metrically in a weak position, yet most readers will give it heavy stress, so the line begins with a slight lean, as if the greatness were already top-heavy. That is my reading of the rhythm rather than anything the poem insists on.
Space matters as well. "From far" is a long, open phrase that stretches the line outward, while "at home" closes it down. The comparative "worse" delivers the sting, because the enemy near at hand is judged more dangerous than the distant one. The poem doesn't say why, but the text invites an inference: an enemy within knows the weak points, cannot be kept out by walls, and sets neighbours against one another. The couplet names no particular quarrel, faction or event, which is why it works as a general maxim and not a comment on one moment.
The rhyme is a final quiet touch. To a modern ear, "none" and "home" make an approximate chime rather than a full rhyme, linked mainly by their closing nasal sounds. The pairing has a neat effect all the same. "None" is emptiness, the enemy abroad removed, and "home" is where the replacement turns up. The rhyme carries the reader from absence to presence in a single beat.
The poem's significance lies in this compression. Herrick is often associated with delicate, playful lyrics, so a couplet this unsparing shows a harder, more observant side, working in the tradition of the epigram, where a whole moral argument is packed into a few words. It lasts because it travels well. "Cities" can stand for nations, but also for any group that has grown large and confident. Anyone who has watched a team, a class or a country turn on itself the moment the common rival disappears will recognise what Herrick is saying, and will notice how little he needed to say it.