Why is "Battle Hymn of the Republic" considered to be a patriotic hymn in the United States?

"Battle Hymn of the Republic" is a well-known hymn written around the time of the United States civil war (1861–1865). It is often performed at patriotic events in the United States, like political party conventions and presidential inaugurations . Because it speaks of battle, it would have been poignant to soldiers during the civil war when it was written, but the text doesn't seem to have much, if any, patriotic imagery – like mention of country, or flags, or an ode to the homeland. Its imagery seems to be mostly Biblical – Christ coming in glory, his triumph over sin, and truth advancing. Paul used this kind of imagery extensively in the New Testament to talk about the war against sin and loyalty to Christ. The closest to USA-specific imagery I can find is the mention of Christ being born "across the sea," but of course that is just as applicable in Canada, Guatemala, Tonga, or Australia as it is in the United States. Why, so many years after the civil war, is this song considered to be a patriotic hymn?