Seventeen words, no punctuation, no rhyme, and still the poem manages to unsettle a comfortable idea of war. Its speaker, who takes the title's tin soldier as a mask, announces that he could join the war, and then, with an odd shrug, decides he would rather be "the enemy / for an afternoon."
The central idea is that "enemy" is a role, not an identity. A toy soldier is always cast on one side or the other, and in any game somebody has to play the bad guy. The phrase "go be the enemy" sounds like a child organising a game in the garden, and that casual tone is the poem's main weapon. In real war, being the enemy is not something you choose or put down again. The speaker's offhand willingness exposes the fact that, from the other side of the line, every soldier is someone's enemy. The poem does not say this outright. It lets the small word "be" imply it.
The phrase "glad as rust" is the strangest and most productive moment. It echoes the stock comparisons we use for contentment, but swaps in something corroded and slow. Rust is what happens to metal left out and forgotten, so the soldier's gladness is passive, almost a surrender. That fits a tin figure, an object whose body is the same material as the simile. It is also unsettling. Rust spreads quietly and eats what it touches, and its colour may remind us of old blood, though that is my inference, not something the poem states. The gladness on offer is therefore the gladness of decay, not of heroism. Nobody here is marching to a trumpet.
The lack of punctuation makes the phrase hover. "Glad as rust" can attach backwards to "I could join the war," as in joining the war glad as rust. It can also lean forward to "I would go be the enemy." The poem never settles the question, and both readings colour each other. Is the gladness about taking part in violence, or about the relief of letting go of a side?
The shape of the poem helps this along. The lines run roughly five syllables, three, eight, five, so the stubby second line works as a hinge between the two sentiments. Nothing rhymes, but there is a quiet echo between "war" at the end of line one and "for" at the start of line four, as if the poem's frame were bent back on itself. There is also a shift in the verbs. The speaker begins with "could," which is possibility, and moves to "would," which is intention. It is a tiny hardening of resolve, and it happens just when the voice is being most flippant.
Then comes "for an afternoon," and the poem turns. An afternoon is a leisure word. It suggests a lazy, bounded stretch of time you can step into and out of, which is exactly what war is not for those who live inside it. Whether this is the speaker's irony, a confession of how easily distant people treat war as spectacle, or an admission that a toy has no way to leave its box, the poem doesn't say. Judging by the tone, there is some self-reproach in it. The speaker seems to know that his casualness is a luxury.
That is the poem's real significance. In a few lines it avoids both glorifying war and lecturing against it. It plants one uncomfortable image, a soldier who is both toy and enemy, rusting happily, and leaves the reader to feel the gap between the game and the thing it imitates. The shortness suits the subject. A longer poem might have explained too much, and here the silence after "afternoon" does some of the work. The reader is left to ask who, in the real version of this game, gets to go home when the light changes.