The main theme here is an elusive one to catch and a difficult one to understand. This is partly because it is unexpected and partly because it contradicts tradition. Thus it cannot be expressed in classic terms, such as, for example, "Patience," "Love and Hate" or "Art and the Artist." This theme seems to be an original one, never given voice to before. It may be that this theme was never true before, thus never expressed. Yet there is a relationship to the idea of belonging.
The elusive main theme may be expressed as: The past is a withered dry leaf that has nothing to do with me or with how I live.
The unstated subtext is Hitler's assault against Warsaw in World War II that killed, imprisoned and exiled so many people who belonged to Warsaw, the symbol for all of Poland. The postcard shows reconstructed Warsaw where belonging is again possible; where bullet holes are no longer visible (though still visible in the ruined stone cottages lining the roads into Warsaw); where high-rises replace razed buildings. A visual subtext underlies the discourse of the postcard picture: "See me: Warsaw?" is attended by (or undermined by the attending binary), "You do not now see that: destruction."
In previous eras, the generations after carnage, assaults, exiles and death were taught to belong to the past, to carry on the memory, seek a return, seek vengeance if possible, even if only the vengeance of belonging, of reclaiming what was once home. The poetic speaker in "Postcard," who may be taken to be identical with the author in this case (though it is by no means true that the poetic speaker is always or even often identical to the author), has an antithetical attitude: he holds exactly the opposite idea from the traditional one of belonging. The speaker, or poetic persona, wants nothing to do with belonging to cherished old Warsaw and "Beloved Ukraine" to which his parents once belonged.
Yet, Warsaw, personified in the last stanza, seems to hold to traditional ideas and is calling the speaker back--as other generations have always gone back--to the "red gables / And a cloudless sky. / On the river’s bank ..." to which he should belong. The speaker refuses to answer--continues to resist--the lone tree by the river's edge that calls him to return and to belong:
On the river’s bank
A lone tree
“We will meet
Before you die.”
The speaker continues to say what is now recognizable as the thematic statement: The past is a withered dry leaf that has nothing to do with me or with how I live. The speaker rejects belonging to Warsaw, to the past, or to the traditions that follow destruction and exile.