A poem is always written by one captive to another.
The aim of the poem is to establish first for the poet and then for the reader a free relationship to the emotion, to liberate the subject from the feeling it implies. The poem must extricate itself, its poet, and its reader, from the history in which they are all implicated.
A poem makes an art of our captivity.
We should not expect poems to free us from political oppression, nor even from personal obsession. These will remain even after we read or write a poem. But the poem will provide us with a moment in which the nature of our oppression or obsession is available to us as an opportunity.*
Ultimately, we are all the captives of our emotions.
Update (15-01-2014, 10:38): The last word in this paragraph has been changed, on a whim. From "emotion" to "opportunity". That totally changes the meaning, but seems more right, and less trivial.