This poem's message is found in the author's use of Irony. The opposite of what one expects occurs.
The reader begins the poem believing that it will be about "Green Berets". Really, the poem is an exposition of the indomitable strength of the simple Vietnamese people.
Initially, the reader believes the Green Berets are "powerful" and in control...yet as the action of the poem progresses, the "frail boy's" resolve makes him a much more fierce fighter in the concluding lines of the poem. We glimpse how the strength of the Vietnamese is such a formidable force.
In the way that the Green Berets committed atrocities they actually strengthened their opposition. Here again we see the poet's masterful use of Irony. As the poem concludes, it turns back upon itself and we see the "Green Berets" as diminished and the Vietnamese as certain to triumph in the impending conflicts.