What would you say the message of this poem is and why?

The primary thematic thrust of the poem is unity. Recognizing that the country is and always has been shattered into opposing camps, the speaker reminds us that these differences are not what have defined America, but those things which have united us. America has always been able to put aside its differences and come together when facing a common enemy threatening and that should be taken as signal that those things binding together must be stronger than those things which threaten to tear us apart.