In my opinion, this is a poem that is trying to oppose the idea of patriotism. Or perhaps I should say it is opposing the idea of blind patriotism.
The author strings together all these patriotic symbols and phrases at the beginning of poem. By doing this, he is giving the usual way that patriotism is seen -- this enthusiastic love of America and patriotic songs, etc.
But then he stops and changes his message -- he starts to be sarcastic. He uses ignorant-sounding language (by gorry, etc) and the idea that people who believe that stuff at the begining also think that it is beautiful to be killed for your country.
At the end, the bit about drinking water implies that the speaker has been rushing nervously through his speech to an audience -- maybe he thinks what he's saying about how we should be patriotic is untrue...