The poem looks at a border from the point of view of someone watching how power works there. It opens with long stretches of time "without a witness," which suggests that much of what happens at a border goes unseen. The state and Customs "assess the foreign," sorting people and things as if they were goods to be inspected, divided and cut up "part by part."
Then the poem shifts to a crow snatching a sandwich with "a cheap cry and fright." The comparison is sharp: officials take what they like, and the noise and scare are part of the method. The image is almost comic, which makes it sting more.
The speaker seems wary and bitter, though the poem never states this outright. The ending is the hardest part to pin down. "Laugh to hear pound / the greatest heart" could mean mocking someone's pain, or it could be a grim, defiant sort of laughter. Either way, the mood is cold, tense and uneasy.